Explain to students that the colored rubber balls represent molecules. Without moving the tub, the molecules remain locked into fixed positions, a "solid state". Now shake the tub side to side. The molecules (balls)begin to move about. The more you shake, add energy, the more they move about, this represents a "liquid state". Finally add more energy to the system, shaking more vigorously, and some of the balls leave the container, some of the molecules enter the "vapor state".