This happens with stuff other than water too! A little over a year ago, I heated my boiled egg in the microwave for a few seconds every morning when I got to work. One morning, I heated the egg as ususal and had actually taken a few steps down the hallway......I went to bite into the egg (just like every morning) and the egg exploded in my mouth.....causing blisters inside my mouth and across the outside of my lips and a few places on my face. Needless to say I'm very careful with anything that gets warmed in the microwave now.