[prewett]

I think that water in a microwave can get super-heated (hotter than 100C) if the surface is smooth, or there are no more little air bubbles to start the boiling. Then, when you put something in, it gives a surface for bubbles that are desperate to form something to form on, and they all do it at once.

I had a spectacular explosion with my glass teapot one time. The microwave at work was in a different room, so I would heat up the water to boiling, and then not hear the bell, so some time later I'd remember, and do it again. Apparently reheating causes the bubbles to basically get used up so there wasn't anything to start the boiling. So the third time I was standing there waiting so I wouldn't forget, and suddenly I hear a BOOM and the pot was half empty with a lot of water outside. I think there was just one giant bubble that eventually formed and blew everything out. (The pot was unharmed)