John,

I have actually had water blow out of a cup in 
the microwave, or even on the counter, after I've 
taken it out of the microwave. I've had this 
happen with gravy, and there was some grease in 
the broth so it really made a mess.

It does sound unbelieveable, but unless you have 
had it happen, it's hard to believe. I've had it happen several times.

Pat Ferguson

At 03:54 PM 8/30/2006, you wrote:
>Phil, from snopes, true, but rare.  I put in for 
>a search string, heating water in a 
>microwave.  After clicking on a link, I snipped this:
>
>Origins:   Exploding water? In a nutshell,
>Too hot to handle
>    yes, water can "explode" in the fashion 
> described above. However, it takes near
>perfect conditions to bring this about, thus 
>"exploding water" is not something the
>average hot beverage drinker who would otherwise 
>now be eyeing his microwave with
>trepidation need fear. Odds are, you'll go 
>through life without ever viewing this
>phenomenon first-hand, and if you're one of the 
>rare few who does get to see it,
>you will likely not be harmed by the experience 
>(that would take your standing right
>over the cup at the instant it happened, and the 
>liquid's bolting up and hitting
>your skin).
>  The Food and Drug Administration (FDA) has advised consumers:
>  This type of phenomenon occurs if water is 
> heated in a clean cup. If foreign materials
>such as instant coffee or sugar are added before 
>heating, the risk is greatly reduced.
>If superheating has occurred, a slight 
>disturbance or movement such as picking up
>the cup, or pouring in a spoon full of instant 
>coffee, may result in a violent eruption
>with the boiling water exploding out of the cup.
>  What Can Consumers Do to Avoid Super-Heated Water?
>  Follow the precautions and recommendations 
> found in the microwave oven instruction
>manuals, specifically the heating time.
>  Do not use excessive amounts of time when 
> heating water or liquids in the microwave
>oven.
>  Determine the best time setting to heat the 
> water just to the desired temperature
>and use that time setting regularly.
>   The e-mailed warning quoted above a model of 
> non-specificity (no names, date, or
>locale; only the detail that the "victim" was 26 
>years old, presumably to enforce
>the idea that this accident could not be blamed 
>on a child's typical lack of caution),
>leaving us able to address only its theoretical 
>aspects.  Nearly every science writer
>who has tackled the topic notes that the "severe 
>superheating" phenomenon is real
>but likely not nearly as common as the message 
>cited above might make it sound.
>  In an article for New Scientist, Richard Barton wrote:
>  A portion of the water in the cup is becoming 
> superheated — the liquid temperature
>is actually slightly above the boiling point, 
>where it would normally form a gas.
>In this case, the boiling is hindered by a lack 
>of nucleation sites needed to form
>the bubbles
>. . .
>  I imagine that by keeping the cup still and 
> microwaving for a long time, one could
>blow the entire contents of the cup into the 
>interior of the microwave as soon as
>you introduced any nucleation sites. It is this 
>sometimes explosive rate of steam
>production that means you should take great care when using a microwave oven.
>   From the
>How Things Work
>  web site:
>  Glass containers are clearly the most likely 
> to superheat water because their surfaces
>are essentially perfect. Glasses have the 
>characteristics of frozen liquids and a
>glass surface is as smooth as... well, glass. 
>When you overheat water in a clean
>glass measuring cup, your chances of 
>superheating it at least mildly are surprisingly
>high. The spontaneous bubbling that occurs when 
>you add sugar, coffee powder, or
>a tea bag to microwave-heated water is the 
>result of such mild superheating. Fortunately,
>severe superheating is much less common because 
>defects, dirt, or other impurities
>usually help the water boil before it becomes 
>truly dangerous. That's why most of
>us avoid serious injuries.
>   From the
>Unwise Microwave Oven Experiments
>  web site:
>  Things are different in a microwave oven. The 
> water gets hot but the container usually
>does not. There are no "boiling-bubbles" 
>triggered by a hot metal pot. Without those
>bubbles to cool it, the temperature of the water 
>rises far higher than 100C°. We
>call this "superheated water." Superheated water 
>is just waiting for some sort of
>trigger which will let bubbles form and allow 
>boiling to commence. If the water becomes
>hot enough, a few bubbles will appear, but these 
>quickly rise and burst, and the
>water isn't cooled much at all. In the microwave 
>oven, even if your mug of water
>is bubbling slightly, don't trust it, since it's 
>temperature has risen so high above
>100C° that bubbles are appearing spontaneously. 
>If some unwitting victim should pour
>a soluble powder into the superheated water, 
>this will carry thousands of tiny air
>bubbles into the water. Each of these micro 
>bubbles expands into a 1cm steam bubble,
>and the result is a huge "explosion" of hot 
>froth. It's just like dumping ice cream
>into root beer, but the froth can be so violent 
>that the hot water sprays into the
>air.
>   If you're worried about explosions over 
> superheated liquid, you can virtually eliminate
>the possibility of its happening to you by 
>simply leaving some non-metallic object
>(such as a wooden spoon or stir stick) in the 
>cup or bowl when you boil liquids in
>your microwave.  It may not be necessary, but it 
>won't hurt anything either.  Peace
>of mind rarely comes so cheaply.
>
>earlier, Phil Scovell, wrote:
>>This is one to look up on snopes I guess but I have heard of this before and
>>things falling apart after being super heatened in the microwave.
>>
>>Phil.
>
>John