Tools: a balloon, two glasses, hot and cold water
At first pour a glass of cold water and a glass of hot water. Put the balloon into the glasses. The balloon in the glass of hot water inflates. The balloon in the glass with cold water deflates. The balloon inflates in hot water because the motion of particles is accelerated due to the heat, and the particles scatter. The balloon deflates in cold water because the motion of particles is slowed down.