Heat is motion - the motion of atoms in matter. The more the motion, the hotter the matter.
What happens if the matter gets hot?
Take a pan of water, for example: the water atoms are always on the move. When you put it on the cooker the atoms move around faster and faster. The water now expands because the moving atoms need more space.
Is it only water that behaves like this?
All materials move more when they are heated but usually you cannot see anything moving because the atoms are so small. And yes, even those atoms that you consist of are constantly moving, and when you have a fever they move just a little bit faster than before.
What kind of movement is that?
You have to imagine this movement as a completely random one - a bit like the dance floor at a very lively party where everybody is hopping about in all directions.
And does the party ever stop?
No. Theoretically that would happen at minus 273.15°C (this is called absolute zero), but it's physically impossible to reach that point. So the coldest possible temperature is a bit above minus 273.15°C.
Did you know?
The famous British physicist J.C. Maxwell once made a joke about this random movement: He suggested that there was a statistical probability that, at some time in the future, all the molecules in a box of gas or a glass of hot water might be moving in the same direction. This would cause the water to rise out of the glass by itself. Of course the chance that this might happen is virtually zero.