What is matter?
Everything in the universe is made of matter: you, your desk, the air and the stars. Matter is made of very, very small particles called atoms. Matter can be in three states.
It has its own shape. The atoms are very close together and hardly move.
It takes the shape of its container. The atoms are close but they slide around.
It spreads out to fill all the space. The atoms are far apart and move fast.
Changing state
When the temperature goes up, the atoms move further apart. When it goes down, they move closer together. That is how matter changes state:
An ice cream melts in the sun.
Water in a pan turns into steam.
Steam makes drops on a cold window.
Water turns into ice in the freezer.
Water is not always a liquid! It is a solid when it is ice, a liquid in the tap and a gas (water vapour) when it boils.