Supercool!
Click to viewHeard about this years ago, but until this recent piece on the Naked Scientists I didn't know how dramatic the effect was. It turns out:
Water can exist in what's called a supercooled state, so it can exist below 0 degrees C in a liquid form. In order to turn it into ice, you have to have the right type of particle, the right surface present to catalyse the ice nucleation process or the ice formation process.
Just a single crystal of ice and the solidification, er, snowballs.