"My 26-year-old son decided
to have a cup of instant coffee. He took a cup of water and put it in
the microwave to heat it. I am not sure how long he set the timer for,
but he told me he wanted to bring the water to a boil. When the timer
shut the oven off, he removed the cup from the oven. As he looked into
the cup, he noted that the water was not boiling. Instantly the water
in the cup "blew up" into his face. The cup remained intact
until he threw it out of his hand, but all the water had flown out into
his face due to the buildup of energy.
this is a fairly common occurrence.
Water (alone) should never be heated in a microwave oven. If water is
heated in this manner, something such as a wooden stir stick or a tea
bag should be placed in the cup to diffuse the energy."
is what a science teacher has to say on the matter:
"Thanks for the microwave
advice. I have seen this happen before. It is caused by a phenomenon
known as super heating. It can occur any time water is heated and will
particularly occur if the vessel in which the water is heated is new.
What happens is that the water heats
faster than the vapor bubbles can form. If the cup is very new then
it is unlikely to have small surface scratches inside it that provide
a place for the bubbles to form. As the bubbles cannot form and release
some of the heat that has built up, the liquid does not boil, and the
liquid continues to heat up well past its boiling point. What then usually
happens is that the liquid is bumped or jarred, which is just enough
of a shock to cause the bubbles to rapidly form and expel the hot liquid.
The rapid formation of bubbles is also why a carbonated beverage spews
when opened after having been shaken."