Big Reaction: Microwaving Ivory soap

The key to this reaction is using Ivory soap. This type of soap has little air pockets throughout (it floats in water). The microwave excites trapped water molecules, transferring heat to the air pockets. The air rapidly expands, creating the foam growths.

I highly recommend watching the full video, below. Steve Mould has a fantastic channel, along with more videos of things inside a microwave. Very cool!

And for the adventurous, How to film in a microwave.