Why did my cup explode when instant coffee was added?

Your Mattness:

The other day I was reduced to making myself a cup of (ugh!) instant coffee. I put a cup of water in the microwave, zapped it for a couple of minutes, took it out, dropped in the coffee powder, and the whole thing exploded. What happened? Did somebody slip dynamite into the Nescafe jar? Was it a warning from the all-powerful Starbucks? Should I be afraid of instant coffee?

-- Jeff, SDSU

Tut-tut, Jeff. Pit bulls. I-15 at 5:00. The Disney juggernaut. Those are things to be afraid of. Exploding coffee cups is a problem easily solved. First of all, not all hot water is created equal. Or equally. When you boil water on the stove, it hots up at the bottom first. The hot water rises, the cold stuff at the top falls, setting up little ocean currents in the pot, so we can see that the water is "boiling." The other clue is all the bubbles of steam that rise from the pot bottom. They form when the heated-up water swishes by a tiny scratch or other imperfection in the pot bottom. Any scratch or bump gives the bubbles a place to form. When enough steam has clumped together, the bubble is light enough to float and it rises to the top and pops. I'm sure you're familiar with the scenario. Leaning lazily on the stove, watching water boil is exactly the kind of mindless activity that hypnotizes us while our little mind voice that sounds strangely like Mom is telling us to get in the shower, get dressed, get to work.

Micro boiling is a whole different thing, and ten times more boring than watching the pot on the stove. Because the waves heat all the water at once, no convection currents form. Not enough water comes in contact with the bubble-starters. We have no visual evidence that anything is going on. It's actually possible to heat the water beyond the boiling point while the water lies there as innocently as Golden Pond. But as soon as you take the cup out of the wave machine and dump in the coffee, you provide jillions of tiny points at which steam can form. Steam forms. Coffee explodes like Mt. Etna. Next time, you pour the coffee in before you nuke the cup.