What you're watching is very "clean" water cooled below its freezing point without freezing. This is known as supercooling and this supercooled state is very unstable. The water hasn't frozen because ice crystals couldn't find something to form around. A little shaking, however, provides the disturbance needed. All it takes is for one ice crystal to form, acting as a seed and starting a chain reaction. Pretty cool huh?

LINKS GOOD ON 2006-12-14

Just as we saw that a liquid can be cooled below its freezing point without freezing, so too can it be heated above its boiling point without boiling. This process is known as superheating. Again, this makes use of very clean, or pure, water (probably distilled), and again it is very unstable. In this case, the addition of a powder provides the perturbation needed to start the water boiling.

LINKS GOOD ON 2006-12-14