I just took a bar of Ivory soap, cut it into quarters, placed the pieces in a bowl, and microwaved for 2 minutes. (Note: It's got to be Ivory soap.)
Why did it expand? Well, unlike other brands of soap, Ivory soap has a lot of air inside. As you heat the soap, the water vaporizes and the air expands, causing the soap to poof out. (See Steve Spangler for more information.)
I had to wait a couple of moments before I could touch it because parts were pretty hot. The texture reminded me of crumbly cotton candy. As we played with the soap, it slowly broke down into a fine powder.
You can still use it as soap... but I'm going to use the soap powder for another reason....homemade laundry detergent!