Danger: Coffee Explosion
You warm up a mug of water for a few minutes in the microwave oven. You take it out, then you dump in some powdered coffee, tea, sugar, etc...

DOOSH! The water explodes in roiling foam, spraying boiling water all over your bare skin, and sending you to the emergency ward. I hate it when that happens. 


Heatin water or coffee in a microwave oven can be dangerous, especially if you use a ceramic mug or clean glassware. Water sometimes "explodes" because the oven heats it to a temperature that's far hotter than the normal boiling point. When this occurs, any tiny disturbance can trigger some violent boiling. The stored energy is released as a steam explosion. This DOESN'T happen when water is boiled in a pot on the stove. The difference: a stove creates small hotspots on the bottom of the pot which are far hotter than 100C, and the hotspots continuously trigger a roiling boil which cools the rest of the water down to 100C. 


Whenever there are bubbles of steam zipping up through the water, those bubbles provide some surfaces which allow the water to make more steam, and this also cools the water down to 100C. In fact, water can only "boil" at places where the water touches a gas. If there are no bubbles, then "boiling" will only happen at the top surface of the water and not down within it. So, when you heat water on the stove, the extreme temperature at the bottom of the pot causes tiny bubbles to form. The boiling water fills the bubbles with steam. The roiling bubbles act to cool the water and keep its temperature at (or below) 100C/212F degrees. 


Things are different in a microwave oven. The water gets hot but the container usually does not. There are no tiny "boiling-bubbles" triggered by a hot stove burner. Without those bubbles to cool it, the temperature of the water sometimes 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 near the top, but these quickly rise and burst, and the water isn't cooled much at all. Even if your mug of water is bubbling slightly, don't trust it, since its temperature has risen so high above 100C that bubbles are appearing spontaneously. If some unwitting victim should pour powder into the superheated water, this will carry thousands of tiny air bubbles into the water. Each of these micro-bubbles expands into a steam bubble large, and the result is a huge "explosion" of hot froth. It's just like dumping ice cream into rootbeer, but the froth can be so violent that the hot water sprays into the air. 


Even more dangerous is to boil water TWICE in a microwave oven. Most containers have tiny scratches in their surfaces, and these scratches contain air. When you heat water, these tiny air bubbles will provide a constant stream of "seed bubbles" so that normal boiling can occur. However, the air in these tiny bubbles within the cracks quickly gets replaced by steam. If you turn off the oven and let the water cool, the steam bubbles in the cracks will collapse and vanish. The seed bubbles are gone. If you now turn the oven on again, the water will superheat. If your luck is bad, the water will superheat to a very high temperature, then explode violently when a single huge steam bubble spontaneously appears. If that bubble should start out at the bottom of the container, the explosion can fling the entire volume of hot water upwards. Sometimes the explosion is so violent that it makes a sharp noise, and can even crack a glass container. 


MOST DANGEROUS:
  BOILING PLAIN WATER...
  IN A CLEAN SHINY CONTAINER (MUG OR PYREX)...
  BOILING IT MORE THAN ONCE (LET IT COOL BETWEEN BOILINGS)...
  COOKING IT EXTRA LONG (STORES LOTS OF ENERGY IN SUPERHEATING)...
  REMOVING IT IMMEDIATELY (NO CHANCE TO COOL DOWN)
  DUMPING IN SUGAR, CREAMER, A TEABAG, ETC. (SUDDENLY ADDS SEED BUBBLES)  
If you avoid the items on this list, you'll probably never see a "coffee explosion." On the other hand, the above list is a "recipe for disaster." DON'T BE TEMPTED TO FOLLOW IT. Instead, here's a simple, HAZARDOUS experiment to try. Wear safety goggles, and don't heat the water for an excessive amount of time. 


Fill a clean mug about 1/3 full of clean water (DON'T FILL IT TO THE TOP!), then heat it for about three minutes in the microwave oven. Now carefully take it out and immediately plunk it firmly onto the tabletop (whack it hard, but not so hard that it breaks.) The boiling water will burst into froth. DON'T BURN YOURSELF! The superheated water acts almost like warm carbonated cola: if you strike the container, it will foam up instantly. 


Another trick: heat up the water to boiling again, remove it from the oven, then immediately insert a dry wooden coffee-stirrer or a wooden popcicle stick into the water. Foosh! The water boils violently. The dry wood contributes a layer of air to the water, and the air fills with steam and expands into a mass of hot foam. 


Another: heat up the water, then pour in a little bit of warm tap water. The water suddenly boils violently! It turns out that the tap water is full of tiny bubbles. If you let the tap water stand around for a half hour before pouring it into the superheated water, all the tiny bubbles will have risen and popped, and the bubble-free water won't trigger any violent boiling. And if you dissolve some salt into your "exhausted" tap water, the water WILL trigger boiling once again ,since the salt contributes invisibly small bubbles. 


Hmmmm. I wonder if de-ionized distilled water in a REALLY CLEAN container will superheat even more than normal? (DANGER, SUPERHEATED WATER CAN BURST OUT OF THE MUG AND SCALD YOU!) I wonder what would happen if we used vacuum-degassed water, or if we put some dishwashing soap in the water... 


SAFETY WARNING: Treat microwave-boiled water with respect. It can "explode" without warning. You can "defuse" it by CAREFULLY inserting a dry wooden stir-stick or toothpick in order to trigger boiling. Don't dump any sugar in a mug of superheated coffee, or the spewing foam *really* gets violent. Don't try to boil liquids more than once, since that removes the tiny bubbles on the container surfaces which act as boiling centers. If you're going to re-heat a previously heated mug of liquid, cook it with a wooden stir-stick which allows it to boil normally. Always allow bubbling liquids to cool for several minutes before adding anything to them (or perhaps reach over and carefully drop in a dry toothpick or a wooden stir-stick to force them into normal boiling mode.) 


PS
Certain types of foods have no bubbles inside, and these foods will superheat and "explode." For example, never cook a whole unbroken egg in a microwave oven. The explosion isn't just messy, sometimes it's violent enough to smash up the inside of your oven or tear off the door. Paste-like canned foods easily superheat since they're too thick to allow streams of tiny bubbles to form. Canned spaghetti sauce is famous for superheating and causing those "BOOMF" mini-explosions that spray the sauce all over the oven. (I wonder if there's any cure for the "Spaghetti-O explosions?" Maybe whip the stuff with a fork before cooking, so lots of air is added? Mix it with dry bread crumbs or other material that's full of air?)