I suspect you were using pure water and it superheated. You were lucky - superheated water from a microwave can explode and cause a burn.
Myth-buster did short vid on it - using a sugar cube instead of tea bag. http://www.youtube.com/watch?v=1_OXM4mr_i0 -----Original Message----- From: Stephen A. Lawrence > Drop a teabag into the cup of water which isn't boiling... Whoa, nelly -- bubbles galore! Now it's boiling! Gosh, what was in the cup before I put the teabag in?
