For our final lab for or sophomore physics majors, we try to show them
some odd things like rattlebacks. We show them how to freeze water in a
vacuum. One uses a shallow dish (1/2 inch of water) on a good insulator
in a vacuum belljar. and start pumping. (This is hard on the pump.) The
vapor pressure drops, and the water boils. The water cools to its
freezing point with the removal of thermal energy.
I saw something surprising, at least to see. The water supercooled,
and while I was watching it froze in about a second. It was slow enough
that I could see a the water-ice boundary move across the dish like
almost like a wave. The dish was then frozen to at least half of its depth.
I understand what happened, but am amazed that the water was
supercooled to a temperature such that so much water would freeze, and
that I happened to be looking at the right moment.