How things work

Wheels are round so they can roll smoothly along the ground without bumping. A round wheel touches the ground at just one spot and keeps turning over that spot, which lets it move easily. A square wheel would get stuck on each corner with every turn.

A car has an engine that burns fuel or uses a battery to make its wheels spin. When the driver presses the pedal, the engine turns the wheels, and the grip of the tyres pushes the car forward along the road. Steering the wheel points the front wheels where you want to go.

A boat floats because it pushes aside a lot of water, and that water pushes back up hard enough to hold the boat. Even heavy boats float if they are shaped to spread out over the water instead of sinking into it. A boat full of air inside floats better than a solid lump.

A plane has long, smooth wings and powerful engines that push it forward very fast. As air rushes over the specially shaped wings, it lifts the plane up and holds it in the air. As long as the plane keeps moving forward fast enough, the wings keep it flying.

A fridge does not make cold, it takes heat away. A special liquid flows through pipes inside and behind the fridge, soaking up warmth from the food and carrying it outside. That is why the back of a fridge feels warm while the inside stays cool.

Flipping a switch is like opening or closing a tiny gate for electricity. When the switch is on, it lets electricity flow through wires into the bulb, and the bulb glows. When you switch it off, the gate opens and stops the flow, so the light goes out.

A key has a special pattern of bumps along its edge that matches one exact lock. When you slide the right key in, its bumps push tiny pins inside the lock to just the right heights. Once every pin lines up, the lock is free to turn and the door opens.

A clock keeps time by counting something that happens at a perfectly steady beat. Old clocks count the swings of a swinging weight, while most clocks today count the tiny, super-fast buzzes of a special crystal. Each steady beat moves the hands or numbers along, one tick at a time.

A balloon floats when it is filled with a special gas called helium that is lighter than the air around it. Lighter things rise through heavier air, so the balloon drifts upward. A balloon you blow up with your breath is full of heavier air, so it falls instead.

A magnet has an invisible pulling power around it that grabs onto certain metals, like iron. You cannot see this force, but you can feel it tug when a magnet snaps onto a fridge or a paperclip. Magnets also have two ends that either pull together or push apart.

A swing goes higher when you pump your legs and lean at just the right moments. By kicking out as you swing forward and tucking back as you swing back, you add a little push to each swing. Timing it right with the swing's rhythm is the secret to going higher.

A bubble is a very thin skin of soapy water wrapped around a pocket of air. The soap lets the water stretch into a smooth ball, and air trapped inside holds the round shape. A bubble pops when its thin skin dries out or gets touched and finally breaks.

When you hit a drum, its tight top wobbles up and down very fast. That shaking pushes the air around it back and forth, and those moving ripples of air travel to your ears as sound. The harder you hit, the bigger the wobble and the louder the boom.

When you blow into a whistle, the air rushes inside and swirls around very fast, making it wobble back and forth. That fast wobbling of air is what your ears hear as a high, sharp sound. Blow harder and the whistle gets louder, because you set even more air moving.

When you pluck a guitar string, it shakes back and forth very quickly, and that shaking makes the air around it move as sound. The hollow body of the guitar catches the sound and makes it louder. Shorter or tighter strings shake faster and make higher notes.

An echo happens when a sound travels out, hits a hard surface like a wall or a cliff, and bounces straight back to your ears. Because the sound has to travel there and back, you hear it a moment after you made it. That little delay is why an echo sounds like a copy answering you.

A lever is a stiff bar that rests on a support called a pivot. When you push down on the long end, the short end lifts a heavy load with far less effort than lifting it directly. The longer the bar on your side, the easier the load feels. A seesaw is a lever you can sit on.

A pulley is a wheel with a rope running over it. Pulling down on one end of the rope lifts a load on the other end, and using several pulleys together shares the weight so it feels much lighter. It also lets you pull down while the load goes up, which is often easier.

A ramp is a slope that lets you raise something without lifting it straight up. You push it a longer distance, but with much less effort at each step, so heavy things feel easier to move. Trading a bigger distance for less effort is one of the oldest tricks in building.

Gears are wheels with teeth around the edge that lock into each other. When one gear turns, its teeth push the next gear along, so they all spin together. A small gear turning a big one makes it go slower but stronger, while a big gear turning a small one makes it go faster.

An electric circuit is a complete loop that electricity can flow around, like a racetrack for tiny charges. If the loop is joined all the way from a battery, through a wire, to a bulb and back, the bulb lights up. Break the loop anywhere, and the flow stops instantly.

A battery holds energy inside as chemicals, ready and waiting. When you connect it into a circuit, those chemicals react and push electricity out through the wires to do a job, like lighting a bulb. Once the chemicals are used up, the battery goes flat, and some kinds can be filled up again by recharging.

An electric motor uses electricity and magnets to make something turn. When electricity flows through a coil of wire, the coil becomes a magnet, and it is pushed and pulled by other magnets so it spins around. That spinning can turn wheels, fans, and the blades inside many machines.

A screen is covered in a grid of millions of tiny dots called pixels, each able to glow red, green or blue. By mixing those three colours at different brightnesses, each dot can show almost any colour. Together, all the dots build up the picture you see, and they change many times a second to make it move.

A plane stays up thanks to a force called lift, made by its wings as they slice through the air. The wing is shaped so that air moving over the top ends up pulling and pushing the wing upward. The faster the plane flies, the more lift the wings make, enough to hold the whole plane up.

A ship floats because of its shape, not just what it is made of. Its wide, hollow hull pushes aside a huge amount of water, and that water pushes back up with enough force to hold the ship. A solid lump of the same metal would sink because it pushes aside far less water.

A helicopter has long spinning blades on top that work like fast, whirling wings. As they spin, they push air downward, and that pushes the helicopter up. By tilting the blades just so, the pilot can hover in place, rise straight up, or fly in any direction.

A hot-air balloon rises because warm air is lighter than cool air. A burner heats the air inside the huge balloon, and that warm, light air lifts the whole thing up into the sky. To come down, the pilot lets the air cool, and the balloon gently sinks.

A bridge carries weight by spreading it out and sending it down into strong supports at each end or in the middle. Some bridges hang from cables, some rest on arches, and some use frames of triangles, but all of them guide the load safely down to the ground. The clever shape does the heavy work.

A triangle is the sturdiest simple shape because its three sides hold each other in place and cannot be pushed out of shape. A square can lean over into a slanted shape, but a triangle stays put. That is why you see triangles inside bridges, roofs, towers and cranes.

A skyscraper stands on a strong skeleton of steel and concrete that carries its weight straight down into deep foundations in the ground. The frame is built to stay stiff, and tall towers are also designed to sway just a little in strong wind instead of snapping. Deep, solid roots keep the giant steady.

An arch is a curved shape that carries weight by passing it smoothly down its curve and out to strong supports on each side. Instead of one spot taking all the load, the whole arch shares it, so it can hold up bridges and doorways for a very long time. Many ancient arches still stand today.

A computer does not really think like you do, it follows instructions incredibly fast. Deep inside, it stores and moves everything as tiny switches that are either on or off, which we write as ones and zeros. By flicking billions of these switches every second, it can do sums, show pictures and run games.

Code is a set of exact instructions that tells a computer precisely what to do, step by step. People write it in special languages the computer understands, and it must be spelled out perfectly because a computer does exactly what it is told, nothing more. Games, apps and websites are all built from code.

The internet is a giant worldwide network that links millions of computers so they can share information. When you open a website or send a message, it is broken into tiny pieces that travel through cables and signals to another computer, then get put back together. It connects almost the whole world in seconds.

Wi-Fi carries information using invisible radio waves that travel through the air. A little box called a router turns your internet signal into these waves, and your phone or tablet catches them and turns them back into pictures and messages. It is a bit like an invisible conversation happening all around you.

A petrol engine makes power with tiny, controlled explosions. It mixes fuel with air inside small chambers, sparks it to make it burn with a bang, and that push shoves parts up and down. Those pushes are turned into the spinning motion that drives the wheels, hundreds of times a minute.

An electric car runs on a big battery instead of burning fuel. The stored electricity feeds a motor that spins the wheels quietly and smoothly, with no engine noise and no exhaust fumes. When the battery runs low, you plug the car in to recharge it, a bit like charging a giant phone.

A solar panel is made of special material that pushes out a flow of electricity whenever sunlight lands on it. The brighter the light, the more power it makes, with no moving parts and no fuel to burn. On sunny roofs and huge solar farms, panels quietly turn light straight into usable energy.

Electricity is made in power stations, then sent across the country through a huge web of cables called the grid. Tall towers carry it long distances, and smaller wires bring it down your street and into your home. The grid keeps balancing how much power is made against how much everyone uses, every second.

A robot is a machine that can sense the world around it, decide what to do, and then move or act on its own. It follows a program written by people, but it can react to what it detects, like a robot vacuum turning when it bumps a wall. Not every machine is a robot, only ones that sense and act.

Robots use sensors, which are like electronic senses, to find out what is around them. Cameras act as eyes, microphones as ears, and touch sensors feel bumps, while some robots even measure distance with beams of light. The robot turns all these signals into information it can act on.

Artificial intelligence, or AI, is a computer program that can do tasks that usually need human-like thinking, such as recognising a face or understanding a question. It does not truly think or feel, but it spots patterns from huge amounts of examples and uses them to make smart guesses. People build it and decide how it is used.

Some computer programs learn by looking at thousands of examples instead of being told every rule. To learn what a cat looks like, a program studies many cat pictures until it spots the patterns they share. Then it can guess whether a new picture shows a cat. The more good examples it sees, the better it gets.

A radio station turns sounds into invisible radio waves and beams them out from a tall antenna. These waves ripple through the air in every direction, and your radio catches them and turns them back into the music and voices you hear. It all travels at the speed of light, so it arrives instantly.

Your phone listens to signals from satellites circling high above the Earth. Each satellite says exactly where it is and what time it sent its signal, and by comparing several of them your phone works out your own spot on the map. It needs at least a few satellites to pinpoint you accurately.

A microwave oven sends invisible waves into the food that make the tiny water bits inside it jiggle very fast. All that jiggling creates heat right inside the food, so it warms from within instead of from the outside. That is why a microwave heats leftovers far quicker than a normal oven.

A touchscreen has an invisible grid of tiny electric sensors just under the glass. Your finger carries a little electric charge, so when you touch the screen it changes the electricity at that exact spot. The device notices which point changed and knows precisely where you tapped or swiped.