"Chuck Tribolet" wrote in message

Water needs some little bit of something for the water vapor bubbles to
form on. If the water is really clean, and the cup too, you can heat
water
to a couple of degrees over 212F because there are no little bits. Then
when you add the coffee, it boils a little.

I've encountered this with the microwave, also. You get a similar response
when freezing pure water -- rain aloft may be super-cooled but still liquid,
lacking what is called "condensation nuclei" - a bit around which to
crystallize. As soon as it impacts an aircraft, it instantly freezes to
the aircraft structure.