What Is Temperature?

Hot, warm, cool, cold

Think about a cup of hot chocolate, a bowl of ice cream, a warm bath and a cold winter morning. Some things are hot and some things are cold. But how hot? How cold? To answer that, we need a way to measure it. That measurement is called temperature.

Temperature tells us how hot or cold something is. It turns a feeling — "that's hot!" — into a number we can all agree on. Instead of saying "the soup is quite warm," we can say "the soup is 40 degrees." Now everyone knows exactly how warm it is.

Measuring with a thermometer

We measure temperature with a tool called a thermometer. You may have had one used on you when you felt poorly! There are many kinds — some have a thin line of liquid that creeps up a tube, and some are digital and show a number on a little screen.

The numbers on most thermometers are in degrees Celsius, which we write as °C. The little circle means "degrees." So a warm summer day might be 25°C, and a cold day might be 2°C.

Here are some useful temperatures to remember:

Temperature	What it is
0°C	Water freezes into ice
20°C	A comfortable room
37°C	The temperature inside your body
100°C	Water boils into steam

You can read more about how water changes between ice, water and steam in changing states: melting, freezing, evaporating.

Why does something feel hot?

Here comes the really clever part. Everything around you is made of tiny pieces far too small to see, called particles. These particles are always moving and wobbling about — even in something that looks completely still, like a metal spoon or a glass of water.

The faster the particles move, the hotter something is.

• In something hot, the particles are zooming and wobbling fast.
• In something cold, the particles are moving slowly.

So temperature is really a way of measuring how fast the tiny particles are moving inside something. When you warm up your hot chocolate, you make its particles move faster. When you put juice in the fridge, the particles slow down. You can learn more about these particles in solids, liquids and gases.

Heat moves from hot to cold

Have you noticed that a hot drink always cools down, and a cold drink always warms up, until they match the room? That is because heat always moves from hotter things to colder things. The fast particles in the hot drink bump into the slower particles around them and pass their movement along, until everything is the same temperature. Heat never flows the other way on its own — a cold thing can never make a hot thing hotter.

Don't trust your hands!

Your skin can feel hot and cold, but it is not a good thermometer. Try this: if you hold an ice cube and then touch a cool table, the table feels warm — because your hand is now so cold! The same table would feel cool if your hands were warm. Your skin only senses the change, so it can easily be fooled. That is exactly why we need thermometers to tell us the real temperature.

Try it yourself! 🧪

Here is a famous demo that proves your hands can be tricked. You only need three bowls and some water.

You will need: three bowls, cold water (with ice), warm water (not hot), and room-temperature water.

1. Fill one bowl with cold icy water, one with warm water (ask a grown-up — it should be warm, never hot), and one with room-temperature water in the middle.
2. Put your left hand in the cold bowl and your right hand in the warm bowl. Hold them there for about one minute.
3. Now put both hands together into the middle bowl of room-temperature water.

What you will feel and why: Amazingly, the same water feels warm to your left hand and cold to your right hand at the same time! Your left hand got used to cold, so the middle water feels warm to it. Your right hand got used to warmth, so the same water feels cold to it.

This shows why your hands cannot measure real temperature — only a thermometer can give the true answer. Your skin tells you which way heat is moving, but a thermometer tells you the actual number.