Here is a video that further explains about heat.
Solids
Solids expand when heated and contract when cooled. However, some solids expand so little that we may not even notice the expansion.
Liquids
Liquids also expand and contract. You can observe your mum when she is cooking. When she fills the kettle to the brim with water and boils it, water will spill out. This simple observation will show you that water actually expands.
Gases
Gases expand and contract too. Let's say an air-tight container with a small opening is cover by a balloon. If you heat up the bottle, the balloon will turn bigger. If you place the bottle in ice water, the balloon will turn smaller. This shows that air expands and contracts.