The instruction manual for my 2 1/2 year-old built-in GE Microwave displays this "Safety Fact" prominently with bold print in a shaded box:

>SUPERHEATED WATER
>Microwaved water and other liquids do not always bubble when they reach the boiling point. they can actually get superheated and not bubble at all.
>Superheated liquid will bubble up out of the cup when it is moved or when something like a spoon or tea bag is put into it.
>To prevent this from happening and causing injury, do not heat any liquid for more than 2 minutes per cup. After heating, let the cup stand in the microwave for 30 seconds before moving it or putting anything into it.

Mary C.