Quote - Actually Dhama you are almost right.

For a snow flake to form you need a microscopic piece of material...a particle of dust for example.

In an experimant I saw in tv a glass vial of pure water was supercooled to well below zero, but ice did not form until an small speck of some dust was added. Then the water turned to ice almost instantaniously.

A quick shock, like a hard tap from a pencil, to the glass will also cause the water to crystalize  when it is below zero degrees. I've even done it with one of my son's teething rings also. The water is very pure and is not always frozen when it comes out of the freezer. When I tap it against the counter it starts to crystalize away from the impact site.

Anyway, there were too many really good images to choose from this time. Made it real hard to vote.