On Thu, 15 Apr 2010 17:01:39 -0500, George Leppla
> wrote:

>http://www.snopes.com/science/microwave.asp
>
>
>A friend sent me the above link. Apparently if the conditions are
>right, you can cause water to explode by heating it in a microwave.
>
>George L

George, this is called "superheating". When water is heated to the
level of boiling, it must have loci (selected places) to focus the
phase shift from liquid to gas. In the laboratory, we use "boiling
beads". which are glass beads with rough surfaces. The microscopic
edges on these beads provide the proper loci for boiling to occur.

If you don't have these "sharp" loci to permit boiling to occur, the
water can get to well over 212 dF, a potential bomb. Shaking the cup
or moving it, of adding a bit of salt or sugar, will precipitate a
sudden mass conversion of superheated water to steam, facilitated by
the sharp points of the salt, or maybe a scratch in the glass of the
cup, or some other locus. In chemists' terms, this is called
"bumping", and it *can* be hurtful.

It's smarter to put the coffee or whatever in first, to ensure that
there is a way for excess heat to be transferred out as gas as it
occurs.

Sorry for any injuries that occurred, but it's a natural thing.

HTH,

Alex