A clear plastic container with 14 rubber balls, representing molecules.
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".
Clear tub of colored rubber balls