After several minutes the balloon expanded enough so that it stood up right. We then placed it on the window sill to cool it down slowly and after several minutes the balloon collapsed. This demonstration showed that gases expand when heated and contract when cooled. The students also used the "Hand Boilers" to further help them see and understand thermal expansion and contraction.
Heat (thermal) energy makes the molecules of matter move faster, causing them matter to take up more space thereby causing the matter to expand. When we take heat (thermal) energy away - make it colder - the molecules of matter slow down and take up less space, thus causing the matter to contract.