In another thread I mentioned microwaving water in my cup at work. WARNING: I have had two recent occasions of superheating the water with somewhat explosive results (one worse than the other). My cup is smooth porcelain and apparently boiling bubbles don't form as easily which leads to the problem. Each time I heard an audible whoosh sound, and one of the times I saw the reaction as it happened. I noticed it looked like the water wasn't fully boiling quite yet, even though the time was sufficient. All of a sudden a rush of boiling hot water flew out of the cup as tons of bubbles expanded at once. The interior of the microwave was all wet and half of the water volume was no longer in my cup. Luckily for me, both explosions happened before I opened the microwave door. I have since learned that placing a wooden stirrer stick or other non-metallic object in the cup enables the regular formation of the bubbles and avoids the problem. I would prefer not flavoring the water with the wood, and understand a glass rod (presumably roughened) would also help avoid the flash boiling problem; although, I haven't tried it.

Flash boiling is a well known phenomenon, you just need something in the water to provide steam bubble nucleation sites. If you're heating the water for coffee, a small pinch of grounds will do. A small glass marble works as well.

Alan

Works well for that office microwave cleaning. All you have to do is wipe up the mess with a couple of paper towels.

I use my plastic stirrer from my AeroPress in my water. I heat all of my water in the microwave that I put in my coffee.

The same thing can happen in the base of a siphon, using a filter that has a metal chain that hangs down into the base provides the nucleation points to prevent this, one reason to use a lock-in glass filter rod.

Squeaky clean, smooth containers increase the odds of something like this happening.

That phenomena is covered in the voluminous number of warnings that occupy about 90% of the owners manual for the microwave. Included are also eggs with shell and potatoes. Things you discover when you do not read the manual ...

Deep round bottom vessels are more prone to this than shallow or flat bottom vessels.

A quick survey of workmates indicates that unfortunately this is a lesser known phenomenon in my small sample.

Quite common, usually covered in detail in college chemistry lab. Same thing happens in long test tubes when heating liquids.

Yeah, I learned it when I was a second grader (in Scandinavia it means less than 8yrs old). But still it was fun until I was told not to do it anymore .

After 40+ years, it might be fun to try some silly stuff again.

Yep, you can blast the top of a Cory siphon pot right up against the ceiling every so often which wakes everyone right up in the morning.