Re: Why does Ivory soap expand in a microwave oven?

Frankly, I'd never heard of this phenomenon, so I tried it.  By golly, it
works!

Remember that all soap contains water, both in the form of water vapor
inside trapped air bubbles (particularly important in the case of Ivory)
and water that is caught up in the matrix of the soap itself.

The effect is caused by the heating of the water that is inside the
soap.  The water vaporizes, forming bubbles; the heat also causes trapped
air to expand.  Likewise, the heat causes the soap itself to soften and
become pliable.  

The result:  the expanding gases form bubbles that stretch the softened
soap, forming a large closed-shell foam.  When the heating stops, the
soap hardens again.

It's actually very similar to what happens when popcorn pops.  
Incidentally, I tried some of the "puff", and found that it is still
soap. Just soap that foamed and then solidified.