This youtube video is circulating the internet via e-mail and depicts a St. Joseph Island man experimenting with water bottles that have been left out in the cold but have not been frozen. He flips the water bottle upside down, and it freezes instantly.
This is a really cool example of a concept known as supercooling. Simply, this is when a liquid can be cooled to a point below its' melting point (or freezing point- depends on how you look at it!).
Ice is a crystal of water, and crystals often need to be seeded by some sort of initial crystal or impurity. I remember trying to get some of my products to crystallize in organic chemistry labs...it was almost impossible without scratching the sides of the glass (which creates an impurity for it to grow onto). I think that the type of water used in water bottles is perfect for this experiment because it contains very few impurities or dissolved ions/minerals. I'm not sure that this would work with tap water, as a mineral (such as...oh I don't know, calcium let's say) would be a good site for crystals to form.
Also, I think that in order for this experiment to work, the water bottle must be left completely undisturbed from the environment (such as in a truck). I think if the wind were able to move the air molecules around within the bottle it would freeze close to 0.
Any other thoughts on the experiment?