Here are a couple of neat videos illustrating supercooling and superheating.
In the first video, a bottle of water has been cooled to below its freezing point. Watch what happens when it's shaken. As soon as the guy shaking it agitates it enough to get an air bubble into the water, it freezes:
In the second video, some water has been heated in a microwave to above its boiling point. Watch what happens when it's disturbed:
Cheers,
Michael