YouTube video that is cool to watch:
Supercooled Water
I once observed a 5-gallon plastic jug with distilled water in it become supercooled overnight in a trailer (it was only 2/3 full). It was left on a bottom shelf during the winter and cooled all night (must have got to 20 F). When I came back in the morning I noted that the water was still liquid; I then gently kicked the container and in a few seconds the water froze solid with an interesting soft crackling sound. Quite cool I thought - Crystal nucleation by a kick.
-Zeno