Look at this. Supercooled water. How can it not freeze when it sits there?
I can’t see those videos. Is there some version of Quicktime I have to download or something?
Read the conclusion at the end of that article, it explains it pretty simply. Quite cool.
Pun intended 
In order for water molecules to crystalize, there need to be some impurities in the water for a crystal to form around. Since bottled water is usually filtered, it has relatively small amounts of impurities. The water can be supercooled well below 0C without crystalizing. Movving or stirring the water is usually enough to move some small impurity around and get the crystal to form. Crystalization is a chain reaction – once you’ve got a small crystal, other ones start forming around it.
A related phenomenon is super-heating, in which water is heated to above 100C without evaporating. This has been known to cause injuries when people remove super-heated water from the microwave, causing it to all boil at once and splash all over.
See also: Supercooling and Superheating.
Supercooled water Googled from various university web sites.
This happens fairly often to me when I put Gatorade bottles in the freezer (maybe 50% of the time). If I leave them in there for a few hours (they will freeze if I leave them there long enough), they are liquid, but a minor shake will cause them to freeze instantly. I agree, very cool to look at.