Many of the demos that I use in teaching chemical reaction are taken from a terrific book called A Demo A Day–Chemical Demonstration Book. The book contains a year’s worth of chemistry demonstrations that are easy to follow and to use. This demonstration shows the concepts of polymers. Styrofoam is an example of a polymer. A polymer is made up of monomers. You can think of a monomer as a Lego piece. When you connect the Lego pieces together you have a polymer. You can take a Styrofoam cup and place it in a petri dish of acetone. Push down on the cup and the cup seems to disappear! The acetone is breaking the bonds between the monomers.
Precautions: Acetone is flammable and needs to be used in a well ventilated area. Wear goggles.
Watch the Google video.