Hmm, dredging up old memories ...
The water molecule, H²O, has the shape:
O
/ \
H H
It's bipolar nature means that when it cools to a solid, it forms a crystal lattice, with six surrounding molecules instead of the expected eight - which is why water expands when it freezes into a solid (the only substance to do so).
Because it needs to form this crystal in order to enter the solid state, it is possible for droplets of water to cool below the freezing point, while still remaining as a liquid. If the droplet has a starting point for crystal formation - then it can begin to build upon that.
If it doesn't then it has to wait for a radom chance alignment to begin crystal formation.
[ 16 October 2001: Message edited by: Checkboard ]