This is cool.
Easiest activity yet!
Only Ivory soap works for this. When you put the soap in an operating microwave, the soap and trapped water absorb the microwave energy and heat up. Eventually the water turns to steam. Steam takes up much more volume than the equivalent quantity of liquid water, and this drives the expansion of the soap bar. In addition, the air in the trapped bubbles expands as it is heated, contributing to the expansion.
This is what Big Boy thought of it: