Re: "Superheated water" from a microwave.

First point 100 deg C (212 deg F) is the bp for PURE water at 1 atmosphere pressure (add contaminants & you change the charicteristics).
As I umderstand the situation (and I add at this point that I haven't seriously looked at Physics for 20+ years), the "exploding water" phenomenon is not so much boiling as "bumping". In the normal coures of boiling, there is a temperature gradient through the liquid from the container up/in, thus the water boils first at the container walls. As the water boils through the bulk it is already boiling at the walls and the vapour bubbles formed at the walls provive a "nucleation point" for the bubbles from the bulk. If I recall the explaination correctly, in a microwave boiled liquid, because the boiling is generated throughout the liquid simulataneously (minimal thermal gradient) there are few (or no) available nucleation sites for bubbles, so, with a liquid in this state, if an object (spoon etc.) is placed into the liquid, it becomes a mass of nucleation sites for bubbles, hence the perceived "explosion".
The simple answer is to put a source of nucleations in during "cooking" (a plastic spoon etc.).,br>

I'm working from a somewhat faulty memory, so I'm ready (and waiting) to be corrected by those who know better.

Regards

Walrus