20 minutes (Score 1)

I had read one of the papers on microwaving your sponges, and since our house was enjoying a bit of a mouse-poop-on-the-counter problem at the time, I figured microzapping the bacteria couldn't hurt. Unfortunately it was an old microwave and there was no "quick minute" button, so in a hurry, I inadvertently set it for 20:00 minutes instead of 2:00. Maybe ten minutes later my housemate says, "my god what is that awful smell," and I run into the kitchen to find the sponge on fire and the microwave still dutifully cooking it. The smell lasted at least a week, and I threw away the microwave. So here's a question...I'm a little hazy on the concept, but microwaves heat stuff by exciting water molecules. Once the water molecules had evaporated what did it heat in the sponge that caused it to catch fire?