# Top 100 Cool Science Facts for Kids!

Eugene is a qualified control/instrumentation engineer Bsc (Eng) and has worked as a developer of electronics & software for SCADA systems.

## The World of Wonder

Why is the sky blue? What is air made of? How many stars are there?

World of wonder fun science facts that every child should know! This article covers space, nature, technology, engineering, elementary math, chemistry, physics and biology.

Science is fascinating and tries to explain how everything in the world and outer space works. Science gives us the answers to questions like "What is electricity?" and "How does an aeroplane fly?" Read on and learn 100 more cool science facts!

## 1. Which Is Heavier, a Ton of Feathers or a Ton of Coal?

This is a trick question and lots of people get caught out. Of course they both have the same weight! However coal is denser than feathers which means a lot of weight is packed into a smaller space or volume. Feathers are less dense than coal but take up lots more space for the same weight.

## 2. Why Is the Sky Blue?

Visible light from the Sun is made up of different colours, in fact all the colours of the rainbow. These colors have different wavelengths. Blue is one of these colours and has a short wavelength. The atmosphere is made up of different gasses we call air, composed of tiny particles called molecules. It also has lots of tiny water droplets floating in it. Blue light can't pass straight through these droplets to our eyes but gets reflected or bounced and scattered backwards and forwards by the gas molecules and droplets, eventually coming out of the sky. The effect is that the sky gets lit up in a blue color.

## 3. Why Do Ships and Ice Float?

The Principle of Archimedes explains why ice floats. This says that the force or push upwards on an object equals the weight of displaced water. Displaced means pushed out of the way. Since ice is less dense than water, the weight of a piece of submerged ice would be less than the weight of water it displaces. So the force upwards is greater than the weight acting downwards and the ice gets pushed to the surface. Ships float also because they displace a lot of water.

## 4. Can We Travel to the Centre of the Earth?

Most of the inside of the Earth is made of really hot melted rock. This part is called the mantle. At the centre of the Earth is the core which is made of solid iron. It would be really difficult to travel to the Earth's centre because it is so far away and all the material would have to be pushed out of the way as we travel. The distance to the centre is nearly four thousand miles. Even building tunnels 20 miles long takes many, many years. Some of the deepest mines are only 2 1/2 miles deep.

## 5. Why Can Birds Sit on Power Lines and Not Get a Shock?

Electricity flows around in a loop. When a bird lands on a power line, electricity can't flow through its body. However if it touched an adjacent line with a lower voltage, electricity would flow from one line through its body to the other line and it could be electrocuted.

## 6. Why Are Things Different Colors?

White light is made up of lots of colors. In fact, all the colors of the rainbow: red, orange, yellow, green, blue, indigo and violet. When white light falls on an object some of it is reflected just like the way a ball bounces off a wall. Other colors in the light are absorbed or taken in by the object and not let back out. So a red object for instance absorbs all colors except red which is reflected. When this red light reaches our eyes we perceive the object as being red. Perceive means how our brain interprets or decides on what is outside our bodies from the information we experience with our five senses. These senses are smell, sight, taste, touch and hearing.

## 7. What Is Sound?

Sound is a vibration of air molecules. When you hit something, it shakes or vibrates really quickly. This shakes the air all around it. The air next to this air also shakes and the shaking continues just like a string of people in a line passing on a message to each other. Sound propagates or travels through the air and eventually we hear it. Sound can also travel through a solid or liquid. Sound has an amplitude and frequency. The amplitude is a measure of the strength of the waves. The frequency is how quickly the sound vibrates

## 8. Can We Hear in Space?

No, we can't because there is no air in space. We call this a vacuum. Without air, vibrations produced by an object or when we speak can't be transmitted through space.

## 9. How Do We Talk to Astronauts in Space?

We can't use sound because it doesn't travel through the vacuum of space and in any case, it wouldn't go far enough. We have to use radio communication. Our voice is turned into electricity by a microphone and then into radio waves or electromagnetic radiation. These waves travel really fast, in fact a signal would go around our planet Earth seven times in one second. When the waves get to the astronauts spacecraft, they are turned back into electricity and sound by a loudspeaker or headphones.

## 10. Why Are Leaves Green?

Leaves contain a chemical called chlorophyll. This chemical turns the gas carbon dioxide or CO2 into stored energy in the plant. All the wood in a big tree comes from carbon dioxide taken out of the air.

## 11. What Is a Light Year?

A light year is the distance light travels in a year. Light travels at a speed of approximately 186,000 miles per second. So in one second it could travel around our planet at the equator over 7 times! In a year, there are 31,536,000 seconds, so the distance light travels is about six million million miles (6 trillion miles). That's 6 with 12 zeroes after it. Light years are used to describe how far away stars are because the number in miles would be too long to write down.

## 12. How Far Is the Nearest Star?

Our nearest star is Proxima Centauri, a red dwarf star just a little over 4 light years away. That's 24 trillion miles. Our Sun is also a star, but it is still really, really far away, in fact 93 million miles. Some stars are so far away that it takes light millions of years to reach us, so we see the stars as they were millions of years ago.

## 13. How Long Would It Take To Get to the Sun if an Airplane Could Fly There?

There isn't any air in space so an airplane couldn't fly to the Sun, but if it could, it would still take over 20 years.

## 14. How Many Stars Are There?

We have estimated that there are 300 sextillion stars. That's 3 followed by 23 zeroes or 300 thousand million, million, million.

This is how we would write that number:
300,000,000,000,000,000,000,000

It is said that there are more stars in the Universe than there are grains of sand on all the beaches of the world. Stars are grouped into clusters called galaxies that can contain one trillion stars. There are estimated to be 100 billion galaxies in the Universe.

## 15. What Is Electricity?

Electricity is the flow of tiny particles called electrons. In some materials such as metals, electrons are not held tightly to atoms and are free to wander. When a voltage is applied to the material, it forces the electrons to flow along it. This flow of electrons is called a current and is measured in amps.

Watts, Amps and Volts Explained — Kilowatt Hours (Kwh) and Electrical Appliances

## 16. What Is Lightning?

When clouds get charged up with electricity during a thunderstorm, the voltage eventually gets too high and the charge has to drain away to the ground. We call this lightning and it's like a giant spark. The sound produced by lightning is called thunder. We hear thunder after we see lightning because light from the flash travels faster to our eyes than the sound. If lightning is far away, it can take many seconds to hear the thunder. The spark in the spark plug of a car is like a mini version of lightning.

## 17. What Is Air Made Of?

Air is a gas, but it's not just one gas, it's a mixture of lots of different types. Most of the air though is made up of the gasses nitrogen, oxygen and carbon dioxide.

## 18. Is Air Heavy?

A cube of air one metre wide (39 inches) by one metre long by one metre high weighs about 1 1/4 kilograms or 2 3/4 pounds.

## 19. Which Gas Do We Breathe?

We breathe air into our lungs and use the oxygen in it. Oxygen combines with glucose in the food we eat to provide us with energy that keep us warm and makes our muscles and internal organs work. Our body makes carbon dioxide gas as a waste product and we breathe this out.

## 20. Is There Air on the Moon?

No, and that's one of the reasons Apollo astronauts had to wear spacesuits that supplied them with oxygen. Other planets such as Mars have an atmosphere, but the atmosphere of Mars has much less oxygen than we have on the Earth.

## 21. Is There Air on the Sun?

No, and the Sun isn't solid like the Earth. The Sun is made of hydrogen and helium which are gasses. These get really hot because the enormous gravity on the Sun is so strong that atoms get squashed together producing nuclear fusion. This makes lots of heat and light that will last for billions of years.

## 22. What Is Gravity?

Gravity is the force of attraction between all objects in space. Even your body has gravity, but it's so small, the force wouldn't attract anything and make it stick. The force of attraction of a magnet is much greater. Gravity is what makes things fall and gives things weight. It also keeps the Moon close to our Earth. Without gravity, the Moon would fly off into space. Gravity also prevents our planet from moving away from the Sun.

## 23. What Is a Force?

A force is like a push or pull. When you push or pull something, you are exerting a force. Exert is another word for apply. The force of air on the underside of an aeroplane's wing gives it lift and makes it fly. A magnet exerts a force on a piece of iron, pulling it. The wheel of a car pushes on the ground and the force on the axle moves the car forwards. When you walk, your feet push on the ground and the ground pushes back. The walls of a building or pillars of a bridge push upwards, and prevent the roof or bridge from falling down. These are called reactive forces. The air inside a balloon pushes on the rubber walls of the balloon, and the force causes the rubber to stretch.

## 24. What Are Magnets Used For?

Magnets are used for lots of things. They can be used to keep the doors of cupboards closed. The needle of a compass is a magnet and always points to the North Pole. Electromagnets are used in doorbells and also in switches worked by electricity called relays. We also use them in motors, electric generators
for making electricity and MRI scanners for seeing inside our bodies

## 25. Are Magnets Really Strong?

Some magnets are very strong. Some of the strongest magnets are used in hospitals in MRI scanners. These magnets are so strong that they can pull metal items out of your clothes or body if they aren't removed beforehand.

## 26. What Is an Electromagnet?

An electromagnet is a magnet worked by electricity. When electricity flows through a wire wrapped many times around a piece of iron, the iron becomes an electromagnet. You can make one by wrapping insulated wire a few hundred times around a nail and connecting it to a battery.

## 27. Why Is Wire Used for Electricity Covered With Plastic?

Plastic is an electrical insulator. An insulator is a material that doesn't conduct electricity. This means that electricity can't pass through it. This keeps you safe from the electricity and also stops the electricity from flowing to where it's not supposed to go. Other materials that are insulators are ceramic (like the stuff in cups and plates), rubber and glass.

## 28. Why Can I See Through Glass?

The answer is really complicated and not even the best scientists are sure. However we know that really good glass transmits a lot of light, but reflects and absorbs very little.

## 29. What Is Glass Used for Apart From Bottles and Windows?

Glass is used to make lenses. Lenses can bend light that passes through them so they're used in glasses to correct people's sight who can't see things clearly that are near to them or far away. Lenses are also used in telescopes and microscopes and lasers.

## 30. What Can I See With a Microscope?

You can see really small things such as bacteria. The most powerful microscopes are called electron microscopes and can see viruses. These viruses, such as COVID-19, are much smaller than bacteria and cannot be seen with an ordinary microscope that works on light.

## 31. How Big Is a Bacteria?

Bacteria are really small and range from about 0.5 to 5 microns long. A micron is one thousandth of a mm. So it would take nearly one thousand bacteria placed end-to-end to measure one mm or 1/20 of an inch. Some bacteria are very big and can just about be seen with the naked eye, which means without a microscope or magnifying glass. These are about half a millimeter long. Bacteria are much bigger than atoms though. Many bacteria are useful and help breakdown organic matter in our environment like leaves from trees and the dead bodies of animals. Some of them even help to digest the food we eat. Others are harmful and make poisons or toxins that can make us sick.

## 32. What Are Atoms?

Everything in the Universe is composed of atoms. They are sometimes described as the building blocks of matter and a bit like Lego because they join together to make bigger things. Everything we see around us is made from them. Atoms are made of even smaller pieces called protons, neutrons and electrons. In some materials, atoms join together to form molecules.

## 33. What Is Matter?

Matter is the stuff in the Universe that we can see. Like water, wood, metal, rock, air, all the things made in factories, even your body. Matter is made up of simpler stuff called elements.

## 34. What Are Elements?

There are about 100 elements. An element is a pure substance that can't be broken down into simpler substances. Some of the names of these elements are iron, copper, gold, carbon, hydrogen, mercury and oxygen. Elements can be solid, liquid or gas. Water isn't an element as it can be broken down into the elements hydrogen and oxygen which are both gasses. We can put the elements hydrogen and oxygen back together again and burn them to make water. When a piece of paper is burned it gets lighter in weight. The black ash left behind is the element carbon, other elements in the paper burn and go into the air.

## 35. What Are Solid, Liquid and Gas?

These are the three forms of matter. Ice is a solid. When it's heated, it turns into a liquid we call water. When we make it even hotter, it turns into a gas we call steam. There are many different types of solids, liquids and gasses. For example, hydrogen and oxygen and chlorine are gasses. You may have smelled chlorine gas from the water in a swimming pool. Gasoline and the metal mercury are examples of liquids and rock, wood, glass and plastic are all solids.

## 36. What Is Rust?

Rust is a compound formed when the elements oxygen and iron join together in a chemical reaction. Only iron and steel rusts. Other metals oxidize or react with oxygen, but the layer of material formed is really thin and protects the metal from further oxidation.

## 37. What Is a Compound?

Compounds are formed when elements combine or join together. They can also be formed when compounds themselves combine with other compounds or elements. This process is called a chemical reaction. Examples of chemical reactions are burning, rusting, breaking up of a liquid with electricity (which is called electrolysis). You can make your own chemical reaction by pouring vinegar on baking soda on a saucer. The baking soda fizzes as it reacts with the vinegar and makes lots of bubbles. The bubbles are filled with the gas carbon dioxide.

## 38. Where Does Carbon Dioxide Come From and How Does it Cause the Greenhouse Effect?

Carbon dioxide is made by all animals including humans. We breathe it out from our lungs. It is also produced when we burn things like coal, kerosene, wood and gas to heat our homes. The engines in cars, trucks, airplanes and ships also use diesel, kerosene and gasoline to make them work and this makes lots of carbon dioxide. Once it gets into the atmosphere, it acts like a blanket and stops the heat we get from the Sun leaving our planet. This is called the Greenhouse Effect. So the Earth gets warmer and this is causing the ice at the North and South Pole to melt. Eventually the water in the oceans will rise. We call this an increase in sea level. The Greenhouse Effect is also affecting the climate all around the world.

## 39. Is the Sea Deep?

Some of the oceans of the world are really deep. The deepest part is called Challenger Deep and is in the western Pacific Ocean. The depth is 36,200 feet or nearly seven miles (11km). This is deeper than Mount Everest is high.

## 40. How Tall Is Mount Everest?

The elevation or height of Mount Everest is 29,029 feet (8,848 meters) 5 1/2 miles (nearly 9 km)

## 41. What's the Difference Between Miles and Meters?

In some countries such as England and the USA, distance is measured in miles, feet and inches. In other countries, distance is measured in meters or kilometers. The system that uses meters is called the Metric System and was invented in France over 200 years ago. Many people like it because everything changes by 10 or a multiple of 10. In these countries, meters is spelled "metres". So there are 10 mm in a centimetre (cm), 100 centimetres in a metre (m) and 1000 metres in a kilometre (km). Scientists, even in the US, use the metric system.

## 42. What Are Metric Units of Mass?

Mass is like weight, but while mass stays the same, weight changes depending on what planet you are on. On the Moon you would weigh less because there is less gravity pulling you down and you could jump the height of a house. Mass is a sort of measurement of how hard it is to push something or slow it down. Mass is measured in kilograms (kg) or pounds.

## 43. What Are Metric Units of Volume?

Volume is the amount of space an object takes up or the amount of space inside an object like a barrel, jug or bottle. Volume is measured in liters (l) or milliliters (ml). A drink bottle contains about 300 ml. An oil barrel holds about 159 liters.

## 44. Where Does Oil Come From?

Crude oil or petroleum is a thick black, brown, yellowish or greenish liquid, extracted from the ground using oil wells on land or at sea. Crude oil was formed when creatures and plants died and settled to the bottom of the ocean. Over millions of years, sand, shells and other stuff collected on top of them. As a result of heat and pressure, this organic material was transformed into oil.

## 45. What Is Oil Used For?

Crude oil is a raw material and isn't really useful until it is processed in an oil refinery. This is like a big factory that boils the oil and separates it into different chemicals. These include gasoline, diesel, kerosene, lubricating oil, wax and bitumen.

## 46. What Is Lubricating Oil?

This is used in vehicles like cars and machines to reduce friction and stop parts wearing out.

## 47. What Is Friction?

Friction is a force that makes it more difficult for things to move. For instance if you try to slide a heavy box, it can be difficult for it to slide along the floor. The amount of friction depends on the material you are sliding and the weight of the object. Heavy things are harder to slide than light things and its harder for something to slide on wood than wet ice. Sometimes we need friction, so that's why we have grips on the soles of our shoes to stop us slipping on the ground and bicycle brakes are made of rubber to slow down the wheels. Often we need to reduce friction so that things can slide easier. So we oil tools and bicycles and put oil in our car engines so that all the moving parts can slide easier. Without lubricating oil, there would be lots of friction in an engine and it would overheat and break up.

## 48. What Does an Engine Do?

An engine is a machine that turns heat into motion. When something is in motion, this means it is moving. Engines burn fuel such as gasoline, kerosene, diesel or gas to make heat. When the fuel is burned, it get's really hot. The hot gas increases in pressure and forces a piston down inside a hollow tube called a cylinder. The piston looks a bit like a bean tin with a rod attached. The rod attached to the piston turns a shaft called a crankshaft, just like the way the pedal on a bicycle turns the cranks. The crankshaft can then run other machinery or turn wheels in the case of a car.

## 49. What Is Heat?

We learned earlier that all matter is composed of atoms. When the atoms shake, or move, we can think of this as heat. The temperature of an object depends on how fast the atoms are moving about or shaking.

## 50. Can Heat Move?

Yes, there are three ways it can move. Heat can be transferred from one place to another by conduction, convection or radiation. When you put a spoon into hot coffee, the handle eventually gets hot. This is conduction and happens when two objects are in contact with each other. When a radiator in a room is turned on, hot air rises and travels around the room. This is convection. For convection to work, we need a gas such as air, or a liquid such as water to move and carry the heat from one place to another. Heat can't travel to us from the Sun by conduction because there is no material in space to conduct the heat or to convect it. However it can reach us as radiation.

## 51. Can We Make Heat?

We can make heat by burning fuel such as gas, coal or oil or by using electricity. Lots of things need heat to work. Engines need heat and power stations need heat to generate electricity. Plants need heat to grow and seeds need heat to germinate. We also need heat in our homes to keep us warm. When making things, heat can be very useful. Blacksmiths heat steel so that it becomes soft like toffee and can be bent and shaped with hammers. In factories metal and plastics are heated and shaped by machines into useful products.

## 52. What Are Seeds?

Seeds are like plant's babies and they contain all the information stored in the form of DNA to make an adult plant. DNA is like a computer program and it tells the seed how to grow, how big to get and what shape the stems, roots and leaves should be. When a seed gets enough heat, oxygen and becomes moist, it germinates. This means the seed splits out of its shell and starts to grow.

## 53. What Are Roots For?

Plants have no legs or mouth so they can't go and find water to drink! They must soak up water through their roots and let it out through their leaves. This is called transpiration. The water is used for several things. It carries nutrients up from the ground, just like we get vitamins from food to help our bodies work better. Also plants don't eat, and get their food from sunlight and carbon dioxide in the air. This is called photosynthesis. Photosynthesis is a type of chemical reaction (which we learned about earlier) and it also needs water to work. Water also helps to cool the plant. The third reason for water is to act as a solution to carry food and nutrients around the plant so that it can grow.

## 54. What Is a Solution?

We learned about solids, liquids and gasses earlier. When you mix a solid with a liquid it can dissolve or it may not dissolve. If you mix salt or sugar or baking soda with water and it dissolves, the result is called a solution. The stuff being dissolved is called the solute and what we dissolve it in is called a solvent. A solution is a type of mixture.

## 55. What Are Other Types of Mixtures?

A solid can be mixed with another solid to make a mixture. When you mix flour and fruit and other ingredients together to make a Christmas cake, this is a mixture. Concrete is a mixture of cement and sand and stone or rock.
Some solids don't dissolve in water. Sand won't dissolve in water, either will flour, and the tiny particles float in the liquid. This is called a suspension. Eventually if the particles are large enough, they will settle out. If the particles are really small and don't settle or settle very slowly, the mixture is called a colloid. Examples of a colloid are milk and paint.

## 56. How Was Rock Made?

The are three types of rock or stone. Igneous rocks, sedimentary rocks and metamorphic rocks.
Igneous rocks were formed when magma (hot melted rock) under the ground cooled. Magma that comes to the surface and flows out of volcanoes is called lava. When this cooled, rock was also formed. An example of igneous rock is granite or basalt.
Sedimentary rocks were formed when skeletons of marine (sea) animals and shellfish settled to the bottom of the ocean. Over millions of years, the enormous weight and pressure squeezed all the stuff together to make rock. Sedimentary rock was also formed when sand and silt settled to the bottom of rivers or oceans and got packed together.
Metamorphic rocks started off as igneous or sedimentary rock but extremely high pressures and temperatures "cooked" the rock, changing it's form. Examples are slate, quartz and marble.

## 57. What Is Pressure?

Pressure is the intensity of a force or how concentrated a force is in a particular area. When a knife is blunt, it won't cut very well even if you force down on it. If you make it sharp it will cut better. This is because the same force is acting down on a really narrow area of the sharpened blade and the pressure is higher. Pressure also applies to gasses, and air in a tyre is under pressure. So is the gas in an LPG tank or water coming out of a faucet. Pressure is measured in bar, pounds per square inch (PSI) or kilo pascals.

## 58. What Are Knives Made Of?

Knives are made of steel. Once upon a time knives and swords were made from iron, but could bend and break easily. People discovered they could add the element carbon to molten iron. This new wonder material was called steel. Steel is harder and tougher than iron and more springy.

## 59. What Is Carbon?

Carbon is an element. Soot is a type of carbon and so is graphite used for the leads of pencils. Diamond is also carbon but looks much different to soot or graphite. It was made deep underground when deposits of carbon were squeezed together under extremely high temperatures and pressures. All these forms of carbon are called allotropes.

## 60. What Are Diamonds Used For?

Diamonds are, of course, used as precious stones in jewellery. They have a lot more uses though because diamond is the hardest material known. Because diamond is so hard it doesn't wear away very quickly. Before people used iPhones, MP3 players and CD players to listen to music, they used to play records which looked like disks of black plastic. The arm on a record player had a tiny piece of diamond called a needle that moved in the spiral track on the record to reproduce the sound. Powdered diamond and chips of diamond are also used on metal disks and drills for cutting and boring holes in stone. When glass needs to be cut, a hand tool with a tiny diamond at its tip is used to score or scratch a line across a sheet of class. The glass can be snapped along the line of the scratch.

## 61. What Is Plastic Made From?

Plastic is made from crude oil and gas. The raw materials are processed in oil refineries and other chemical factories (chemical plants) and made into chips of plastic. These chips can then be melted down and the melted plastic squirted into moulds to make all sorts of different products. Plastic sheet is made by blowing air into hot, soft plastic so that it blows up like a balloon. Then it can be cut up into sheets and made into plastic bags.

## 62. How Many Types of Plastic Are There?

There are about seven types of plastic that we come across in our everyday lives. These include polythene, polystyrene, polyester, PVC, polycarbonate, polyurethane and polypropylene. Plastics have replaced a lot of materials that were used years ago like metal, glass and wood.
PVC, Polypropylene and Polyethylene - How Plastics Are Used in the Home

## 63. What Is Metal?

Metal is a material that becomes shiny when polished and has lots and lots of useful properties. It conducts (carries) electricity and heat very well and many metals can be hammered into different shapes (it is malleable) or stretched like chewing gum (it is ductile). Metals like steel can also be made springy.

## 64. What Is Metal Used For?

Metal is used for making parts for machines, the bodies of cars and other vehicles, pipes for carrying water and heating gas, cables for conducting (carrying) electricity, nails, nuts, rivets, bolts and other fasteners for joining things together and steel beams called girders, used in the construction of buildings.
These are the names of some common metals you can find in your home:
Iron, steel, stainless steel, copper, brass, aluminium, tin, gold, silver, zinc and nickel.

## 65. What Is Heating Gas Made From?

There are several different flammable gasses used for heating homes, powering vehicles, cooking, and blow torches. Flammable means that something burns really easily. These gasses are made from raw gas or crude oil extracted from the ground or sea using big structures with long drills and pipes called oil rigs. The most common gas fed by pipe to our homes is methane. Propane and butane are two other types of gas, supplied in gas bottles (sometimes called cylinders). These are also called liquid petroleum gas (LPG or LP). None of these gasses have a smell when they are made. This would be very dangerous if there was a gas leak. So an artificial smell which is really distinctive and smelly is added so we can tell instantly if there is a leak.

## 66. How Do We Smell Things?

Our noses have thousands of nerves that connect up to our brain. Each of these nerves is like a sensor that can detect different chemicals. Most substances like food, flowers, wood, soil and other organic materials give off volatile chemicals. These chemicals are light and float easily through the air. When they get into our noses, they dissolve in the mucous lining that coats the inside. Each chemical triggers a different nerve. Because a particular smell can be a combination of hundreds of different chemicals, this is what makes each smell unique.

## 67. What Is a Sensor?

A sensor is a device that detects things like temperature, pressure or light intensity and turns the level or size of that property into a signal. Usually that signal is an electric voltage. The voltage can then be measured by a meter that displays the value of that property (e.g. temperature in a room). Sensors could also be connected to a computer or machine or other system. So for instance in a heating system, a temperature sensor controls whether the heating needs to be turned on or off. An oil level sensor in an engine senses if the lubricating oil level is too low. The fuel gauge in a vehicle uses a sensor to detect the fuel level in the fuel tank. Another type of sensor is called a proximity sensor. This is what stops the conveyor belt in a store when your shopping gets to the till. These sensors are also used for automatic doors in stores and turning on lights at night when you walk by them.
There are hundreds of different types of sensors, measuring and detecting all sorts of things.

## 68. What Is a Computer?

A computer is a system used to process data. The earliest computers were huge, took up a whole room of space, weighed tons, consumed a huge amount of electricity and cost thousands and thousands of dollars. These computers were specially designed to do calculations for the army and for solving secret codes. A laptop computer is thousands of times more powerful than these first computers. Originally computers were designed to just perform mathematical calculations (just like we now use a scientific calculator), or store records of data such as names and addresses. However computers are now used to do a multitude of different tasks like image processing, word processing, displaying Internet webpages and computer aided design (CAD). We interact with some computers using a keyboard and mouse or touchscreen. Other computers are built into systems or machines and can interact with sensors and provide output to control the machine or system. In your home you have lots of these special purpose computers called microcontrollers. They are used in devices like washing machines, burglar alarms and TVs.

## 69. What Is a Ton?

A ton is a measurement of weight. It means different things in different countries. In the United States, a ton is 2000 pounds (short ton). In the United Kingdom, a ton is 2240 pounds (long ton) . The tonne is a metric measurement and that tonne is 1000 kg. A cube of water with sides one metre long, weighs one metric tonne.

## 70. Is Speed a Measurement?

Yes, it is a measurement of how far an object travels in a certain length of time. For instance if a car travels 50 miles in a time of one hour, the speed is said to be 50 miles per hour (MPH).

## 71. Do Some Things Travel Really Fast?

Yes. This is a list of things that travel really fast:

• Sound travels at a speed of 767 miles per hour, 1130 feet per second or 343 metres per second.
• A rifle bullet can travel at up to four times the speed of sound.
• A rocket must travel at 25,020 miles per hour or about 7 miles per second (40,270 km/h) so that it can orbit the Earth. To escape from the Earth's gravity so it can travel to the Moon and planets, it has to travel faster.
• Light travels at a speed of approximately 186,000 miles per second or 300 million metres per second. This is the fastest speed. Nothing can travel at the speed of light although its speed can get closer and closer but never actually reach light speed. A beam of light could travel over 7 times around our planet Earth in one second.

## 72. What Are Some Facts About the Earth?

• The Earth is one of eight planets circling or orbiting the Sun.
• The distance from the Earth to the Sun is 93 million miles or 149 million kilometres.
• The Earth has a diameter of 7918 miles or 12,742 km.
• The weight of the Earth is estimated to be 6 quadrillion kg. That's 6 million, million, million, million kg. If you write out the number, it looks like this:

6,000,000,000,000,000,000,000,000
• The age of the Earth is about 4.5 billion years. This is what the number looks like:

4,500,000,000
• Our Earth is nearly 3/4 covered by water. So there's more ocean than land.

## 73. Which Is the Biggest Ocean?

The Pacific Ocean is the largest ocean on Earth and it separates the continents of Asia and Australia from North America and South America.

## 74. What Is a Continent?

A continent is a large area of land that can include several countries. Continents don't necessarily have to be like big islands surrounded by ocean, although some are. People just decided to give names to large landmasses. There are 7 continents and their names are:

• North America
• South America
• Europe
• Asia
• Africa
• Antarctica
• Australia (Oceania)

Three of the countries in the continent of North America are Canada, the United States and Mexico, but there are several more.

## 75. Do Continents Float on the Ocean Like a Ship?

They don't float on water, but they do float and move on the Earth's mantle. This is called continental drift. The continents form the outer skin of the Earth called the crust that extends to about 40 miles (65 km) deep. Below this is the mantle which gets softer and softer at depths closer to the centre of the Earth. Lava that flows out of volcanoes originated as liquid rock or magma which came from the mantle. Continental drift happens really slowly and continents move at about the same rate as your finger nails grow.

## 76. How Do Volcanoes Form?

Volcanoes occur where there's a crack or rupture in the Earth's crust. The crust is made up of 17 pieces of crust called tectonic plates which move apart (diverge) or move towards each other (converge). At the boundary (edge) of these plates, magma is able to squeeze upwards through the crack and volcanoes form as the magma escapes and becomes lava. Over hundreds or thousands of years, lava builds up into a mound and forms volcanic mountain peaks.

## 77. Are Earthquakes Like Volcanoes?

No, but they usually happen at the boundaries of tectonic plates, just like volcanoes. When plates push towards each other, pull away from each other, slide against each other or push under each other, pressure or tension can build up. Suddenly this can be released and the plates can give a jerk which causes the land to vibrate and waves to ripple outwards, just like ripples travel outwards from a stone thrown into a pond. This is like when you try to slide something heavy along the floor and have to push it really hard. Initially it doesn't move, but all of a sudden it can slip and move and then stop again. In some places tension builds up over years or hundreds of years and eventually the land can slip suddenly, releasing tension. The shaking of the ground is what causes buildings to fall down and people to lose their balance during an earthquake.

## 78. What Are Tension and Compression Forces?

People can get tension or stress headaches, but in science when we talk about tension, we mean a type of force (which we learned about earlier). When you pull the end of a spring, the steel in the spring pulls back. This is because all the atoms in the steel attract each other. The harder you pull, the harder the spring pulls back. Other examples of tension are the force in a steel rope when a crane lifts a heavy load or the tension in the cables of a suspension bridge (like the Golden Gate Bridge in San Francisco). People called engineers must design these cables so that they are strong enough to withstand the tension force without snapping.

The opposite to tension is compression. Tension happens in a material when something is pulled or stretched. Compression occurs when something is squeezed. Some materials such as steel are used in construction because they are good at withstanding tension forces without snapping. Other materials such as concrete and stone are good at being compressed but they would snap if they were bent or stretched. However we can have the best of both worlds by putting steel into concrete when it is being made. This makes the concrete strong if it's compressed or stretched. You may have seen construction workers doing lots of work with steel when a building is being erected. They are putting reinforcing bar (rebar) into place before concrete is poured into molds.

## 79. How Are Bridges Made?

There are lots of different types of bridges and humans have been building them for thousands of years. The earliest bridges were probably made by placing trunks of trees across a gap or stream that people wanted to cross. Bridges then got more complicated and people started building them from stone and lengths of timber. The wood was made into frames made up of lots of triangles to make them strong. People also discovered that if a shape called an arch was used, less stone would be needed and the arch could allow the water in a river to flow through it. An arch shape is really strong too because the weight of all the stones above it make the parts of the arch squeeze tightly together so they don't fall down. Long bridges could be made of lots of arches side by side. When iron and steel were first used for construction of bridges, they were also made into arch shapes. Modern bridges are made of concrete and steel. Big blocks of concrete that rise high out of a river called piers are made on the base or bed of a river. The foundations or base of the piers extend deep down into the base of the river. A bridge that has a long span or length may need ten or lots more piers to support its weight. Some bridges like the Golden Gate Bridge don't need as many piers and the roadway is hung from steel ropes. These are called suspension bridges.

## 80. What's a Mold (Mould)?

A mold is like a tool that we use for shaping things we need to make. In the kitchen we pour Jell-o (jelly) into a mold and when it sets, it's shaped like the mold. Molds are used in construction for shaping pavements, walls of buildings and columns of bridges. In factories, they are used in the manufacture of numerous things including building parts like blocks and bricks, plastic and metal parts for machines and foodstuffs like chocolates and biscuits. Sometimes pouring stuff into molds doesn't work so well because the material is too sticky and would take ages to flow into small gaps and it's better to squeeze or push it into the mold under pressure. This is called injection molding. This is often used for making hollow things like plastic toys and plastic plumbing fittings for connecting pipes.

## 81. What Is Food For?

We eat food for several reasons:

• It's necessary for our bodies to grow and mature into adults.
• Once we mature, food is still necessary to replace cells that die.
• Food gives us energy to do our daily tasks.
• Nutrients contained in food are essential for our organs to function properly

When we eat, our food is broken down into simple chemicals by our digestive system. These are like basic building blocks. Then these simple molecules are reassembled into more complicated molecules to replace worn cells and chemicals, that allow our body to function properly. This is a bit like taking a Lego model that you've built, apart again so that you can re-use the blocks.

## 82. What Are Fat, Protein and Carbohydrate?

You may have seen these words on the packaging of food. These are the three components or nutrients of food we eat, but there are different proportions or percentages of fat, protein and carbohydrate in every food.

• Fat is used for insulating our organs and keeping us warm, storing energy that we can use later and protecting our vital organs.
• Protein is used as the raw material for building muscles and also for providing energy for our metabolism (the working of of all the parts in our body).
• Carbohydrate is a fuel source for metabolism. If we eat too much, the excess is converted to fat and used for storage of energy in our body . The more we eat, the more fat is stored so that eventually we become overweight or obese.

## 83. What Does Percentage Mean?

Per cent is like fractions and a way of explaining how much something is a fraction of something else.

Imagine you have a round cake and you cut it up into 100 equally sized pieces. If you give someone 25 of those pieces, the fraction of the cake you give them is 25/100 which can be simplified to 1/4. The 25 parts out of 100 can be written as twenty five per cent or 25%.
Now imagine you cut the cake into 4 equal sized pieces and give someone one piece. You've given them 1/4 of the cake but 1/4 is the same as 25/100 which is still 25%
25% means the same thing as "twenty five hundredths" or as a fraction 25/100.
To go from a percent value to a fraction, you write the value over 100 as a fraction

e.g. What is 10%?
10% = 10/100 = 1/10 or 0.1 as a decimal

e.g. What is 3% of 250?

3 % = 3/100
3/100 x 250 = 7.5

To go from fractions to percent, multiply by 100

e.g. What is 4 parts out of 5 in per cent?
4/5 x 100 = 80%

## 84. Can We Write All Numbers as Fractions?

We write fractions using a line with a number called the numerator on top and a number called the denominator on the bottom. The numerator and denominator are integers and integers are the numbers we use for counting.
So a fraction could be 1/3 or 1/4 or 13/17.

We call these fractions rational numbers because they are the ratio of two integers

Some numbers can't be written as a fraction. These are called irrational numbers. An example is pi (π) which is the ratio of the circumference to the diameter of a circle. Pi is approximately 3.1416. Another example of an irrational number is √2 which is the square root of 2.

## 85. How Do We Use Pi?

The number pi can be used to find the circumference of a circle. The circumference is the distance all around the circle. If you draw a line through the centre of a circle from one side to the other, this is the diameter. If you multiply the diameter by pi, this gives the length of the circumference.

Example: The diameter of a circle is 2. What is the length of the circumference?
Circumference = diameter x pi = 2 x 3.1416 = 6.2832

## 86. What Does Square Root Mean?

The square root of a number is the number you multiply by itself to get that number.

So the square root of 4 is 2 because 2 x 2 = 4
The square root of 9 is 3 because 3 x 3 = 9
The square root of a number is written like this

√16

## 87. Can All Numbers Be Written as Decimals?

No. We can write a half, 1/2 as 0.5 in decimal form.
We can also write one quarter, 1/4 as 0.25 in decimal
One tenth, which is 1/10 is 0.1 decimal
These are called decimal fractions.

Some numbers like one third, 1/3 cannot be written in decimal format using a fixed number of digits. That is because all the digits needed to represent the fraction would go on for ever.
So 1/3 = 0.33333333...... for ever.
We call these decimals recurring decimals because the digits keep recurring or repeating.
So one seventh 1/7 = 0.142857142857142857.... and so on.

## 88. What's the Biggest Number?

There isn't one! That's because no matter how big a number you can think of, you can just add 1 and get a bigger number. You may have heard of infinity, but it's not really a number. We just use infinity in math when we are working out problems. We say that a number "tends towards infinity" which means that it gets as big as we want it.

## 89. Is Space Infinite?

Does space go on for ever and is it infinite in size? We don't really know. Some scientists think it does and you can travel for ever in a space ship and never get to the edge of space. Others think that space is curved somehow and you travel outwards but eventually get back to the point you started from. This is like travelling around the Earth but since Earth is a ball or sphere, you eventually get back. However for this to work, space would have to be curved in four dimensions.

## 90. What's a Dimension?

A dimension is a way of measuring something. So if you have a straight line, it has one dimension. A square has two dimensions, its width and length. A cube is a solid shape that has three dimensions, its width, its length and its height.

## 91. What Are Solid Shapes?

These are shapes that have three dimensions. Examples of solids are cubes, spheres, cones, cylinders, torus (doughnuts) pyramids and prisms. A rectangular prism is a cube that has different length sides.

## 92. What Are Examples of Solid Shapes?

• Cubes and Rectangular Prisms. Boxes, tanks, bricks, lengths of timber, dice
• Cylinders. Tanks, pipes, chimneys, wheels
• Spheres. The Earth, balls, gas tanks, ball bearings
• Pyramids. The Pyramids in Egypt
• Triangular Prisms. Pieces of Toblerone
• Cones. Funnels, ice cream cones
• Torus. Ring doughnut, hula hoop, rubber o-ring

## 93. Why Do We Use Wheels?

We use wheels to reduce friction. If we didn't have wheels or rollers, vehicles and other things would have to be slid along the ground and it would take a lot of force to do this.

## 94. What Else Are Wheels Used For?

Wheels are used on cars, buses, trucks, trains and trailers, but they're also used in the form of pulleys for lifting things and as gears in machines. Engines have lots of pulleys and gears that turn really quickly.

## 95. What Does a Gear Do?

Gears are like wheels with teeth all around the edges that can fit into each other. If you have one gear turning one way, a second gear that meshes with it (the teeth fit into each other) will turn the other way, so gears can be used to reverse direction. If one gear is big and it drives a second gear that is small, the second gear turns faster and this can be useful. We use gears in clocks to make the hour, minute and second hands turn at different speeds. A more complicated thing that gears can do is increase torque or turning force. We can do this by getting a small gear to turn a bigger gear. The bigger gear turns slower, but torque is increased. Gears are used on bicycles and cars so that the engine can give the wheels a lot of torque to get the bicycle or car moving easier from a standstill.

## 96. How Do Clocks Work?

Older clocks used methods like the burning rate of candles with marks on them or the drop in water level in a container as water dripped from it as a way of measuring or indicating time. The problem was that these events could happen at a varying rate and it wasn't very accurate. For instance the rate at which water empties from a container slows down as the water level drops and also if the temperature of the water changes on hot days. The problem was solved by designing clocks which employed something in their mechanism that happened at regular intervals with the interval having an accurate and fixed length that was constant and didn't change with time.

Most modern clocks and watches or timepieces use a component or part inside called a harmonic oscillator, which has a fixed length time period. A swing in a playground is an example of a harmonic oscillator because when pushed, it oscillates or keeps moving forwards and backwards repeatedly. The length of time it takes a swing to move forwards from the rest position with the chains hanging downwards, then backwards, then forwards again to the rest position is called the period. In clocks and watches, we use much smaller things such as pendulums, tuning forks, quartz crystals, spiral springs or the motions of electrons as the harmonic oscillator. Each of these components oscillates or vibrates repetitively and this motion can be used to drive gear wheels and hands of a clock, or the events can be counted electronically and displayed on a digital display as a time in hours, minutes and seconds. Electronic timepieces are much more accurate than mechanical ones because the period of the oscillator is not affected by temperature or friction which can lengthen or shorten the period.

## 97. What Is a Tuning Fork Used For?

A tuning fork is a u-shaped bar of metal with a handle. When struck against a hard surface like the edge of a table, it vibrates and makes a pure sound of a certain frequency. This can be used to tune musical instruments. To do this, the instrument is adjusted so that it produces the same tone or frequency as the tuning fork.

## 98. How Does a Musical Instrument Make a Sound?

There are several types of musical instruments and they make sound in different ways. However the sound is always produced by vibrations of parts of the instrument. There are four main categories:

• Stringed Instruments. These have strings made of different metals such as steel or brass or plastic. Sound is is made when the strings are hit with hammers operated by keys (e.g. a piano), plucked with fingers (e.g. a guitar or harp) or rubbed with a bow coated with resin (violin or cello). The strings vibrate and make sound.
• Woodwind Instruments like the flute, pipe organ and clarinet have tubes through which air is blown. When air hits a sharp edge or alternatively a reed in the instrument, it vibrates and causes all the air in the tube to vibrate and setup what is called a standing wave. The tone or frequency of the sound can be changed by changing the length of the tube.
• Brass Instruments like trumpets, tubas and French horns are like woodwind instruments. Air is blown through them, but instead of a reed or sharp edge vibrating, the player's lips vibrate and this makes the air in the instrument vibrate also.
• Percussion Instruments. Sound is made by hitting the instrument with sticks or hammers causing it to vibrate. Some examples are drums, xylophones and cymbals.

## 99. How Do We Speak and Make Sound?

Just like a stringed musical instrument, we have vocal chords in our throat that vibrate when we blow air through them as we speak or sing. The vocal chords just make tones in the same way as an organ pipe makes a continuous sound. To create sounds that can be understood by people, we modulate or shape the sound by moving our lips, teeth and tongue. We do all this unconsciously without even thinking about it.

## 100. How Many Teeth Do We Have?

Adults have 32 teeth, 16 on top and 16 on the bottom. Some of the teeth called incisors at the front of our mouth are for biting off chunks of food. Canine teeth are for tearing food and in some animals like dogs, these are really long and sharp. Once we bite off pieces of food, we chew it into pulp using molar teeth located at the sides of our mouth.