Often times when matter is heated it will expand. This is called thermal expansion. This happens because the particles that make up matter gain or lose energy depending on temperature. When particles are heated up they gain energy and will move faster and farther apart. Therefore matter with a higher temperature generally has more volume. When the temperature cools the particles lose energy, move more slowly and move closer together. Generally matter with a cooler temperature will have less volume.
Example:
Blow up a balloon with air from your lungs which is fairly warm in temperature. Tie the balloon off and observe the size of the balloon or measure the circumference of the balloon. Put the balloon in the freezer for 15 minutes. Observe the size of the balloon after it was in the freezer and compare it to the size it was before you put it in the freezer. It should be smaller because the cold air in the freezer caused the air molecules to contract and therefore the balloon has less volume.
