This afternoon my youngest son pulled out his Klutz book, called Boom, Splat, Kablooey and did one of the experiments in it, which was to put a fourth of a bar of Ivory soap in the microwave to watch it expand. It was very entertaining.
Notes: Since I've been asked some questions about this in comments and some of you might try it, I'll give some directions. Cut the bar of ivory soap into fourths. Put 1/4 on a paper plate and put in microwave for 1 minute. It turns into this foamy stuff that breaks easily into little bits, so you may end up sweeping some of it up but it's not terribly messy. By foam, I mean a dry foam, more like styrofoam than anything else I can think of.