What Is Energy?

Look around and watch things happen. A ball rolls. A kettle heats up. A lamp lights the room. A radio makes a sound. A puppy grows into a dog. Every single one of those is a change — and behind every change there is the same secret ingredient.

That ingredient is energy: the get-up-and-go of everything. Energy is what makes things happen or change — move, heat up, light up, make a noise, grow. Take the energy away and the world simply stops. The ball won't roll, the kettle stays cold, the lamp goes dark, and you can't so much as wiggle a finger.

You can't see it — but you can always spot what it does

You can't hold energy in your hand like a marble or keep it in a jar. It isn't a thing at all. But you can always catch energy in the act, because you can see the change it makes:

A ball only rolls when something gives it a shove full of energy.
A torch only glows when the battery inside is passing on its energy.
A drum only bangs when your hand hits it and hands over some energy.
A seed only pushes up into a plant when sunshine has poured energy into it.

Wherever something is moving, warming, shining, sounding or growing, energy is quietly doing the work. No energy, no change. It really is that simple.

Energy comes from a source

Energy never appears out of nowhere. It always comes from somewhere, and we call that somewhere an energy source. Every busy thing you can name is drawing on a source:

Food is the source for you and every animal — that's why you get hungry.
Fuel like petrol, wood or gas is the source for a car, a campfire or a cooker.
A battery is the source for a torch, a toy, a phone or a remote.
The Sun is the source for plants — and, as you'll see, for almost everything in the end.

Different jobs need different sources, but the rule is always the same: no source, no energy — and no energy, nothing happens.

Energy can be stored, then let out later

Here is a clever trick that energy can do: it can be stored up and saved for later, then passed on when it's needed. A source is really a store of energy, waiting.

A battery holds a store of energy until you switch the torch on.
Food keeps energy tucked inside it until your body uses it to run and think.
A stretched spring or a pulled-back catapult holds energy until you let go — and then it lets it all out in a snap.
Fuel stores energy quietly until it is burned.

That's why a wound-up toy can sit still on the shelf and then suddenly whir across the floor: it was holding its energy all along, ready to hand it over.

Ever seen a wind-up torch or a clockwork radio? There's no battery inside. Instead you turn a little handle round and round, and every turn tightens a spring coiled up in the box. You are feeding your own energy — from your last meal — straight into that spring, which stores it.

Then let go, and the spring slowly unwinds, passing its stored energy on to make the bulb glow or the music play. Wind it more and it lasts longer, because you packed more energy in. It's the same idea as a battery, only you can watch it filling up with every turn of the handle.

The more you put in, the more can happen

Energy comes in bigger and smaller helpings, and the amount matters. Put in a little energy and you get a little change; put in a lot and you get a big one. A gentle throw sends a ball a short way; a mighty throw sends it soaring. A weak battery gives a dim glow; a fresh one gives a bright beam.

Let's fill a toy car's tank and see. Slide the energy in the tank up and down: the more energy you put in, the fuller the tank glows — and the further the car can travel. With an empty tank it can barely move at all.

Follow the energy back and you reach the Sun

Now for the biggest idea of all. Follow almost any bit of energy back to where it began, and you keep arriving at the same place: the Sun.

Plants soak up sunshine to grow. You eat the plants (or eat animals that ate the plants), so the energy in your dinner started as sunlight. Even the petrol in a car and the coal in a power station came from plants that grew in the sunshine long, long ago. The wind that spins a windmill is stirred up by the Sun warming the air. So when a car zooms, a lamp shines or you race about the playground, you are all really running on sunshine.

Trace it back. Right now your energy comes from your breakfast. But where did your breakfast get its energy? If you had toast, that's wheat — a plant. If you had milk or an egg, that came from an animal that ate plants. And every plant on Earth built itself out of sunlight.

So the energy powering you as you read this set out from the Sun, streamed across space, was caught by a plant, packed into your food, and is now bouncing you around. You are, quite literally, running on sunshine.

Energy isn't a thing you can hold. It's not a substance like water or sand — it's the ability to make change. You never see energy itself, only the moving, heating, shining and sounding it causes.
"Having energy" isn't only about feeling lively. A quiet, still battery is packed with energy. A tank of cold petrol is full of energy. Something doesn't have to be jumping about to be storing lots of it.
Batteries and food don't make energy from nothing. They store energy that came from somewhere else, and then hand it on. Nothing anywhere can create brand-new energy out of thin air.