This science experiment has been on my to do list for years. We finally did it yesterday. I gathered all of the bars of soap in our house and the bar of Ivory I bought just for this experiment.
I then asked what would happen if we put the Ivory soap in the microwave. Reagan hypothesized that since it has air in it, the soap would expand - bright girl. She also knew that since the other soaps do not have air in them, they would melt in the microwave. We couldn't end with the hypothesis, so here is the Ivory in the microwave.
Later, for the sake of comparison, we had to microwave a marshmallow, of course.