That's a nice deep question.
When the atoms or molecules in a liquid are cooled, they don't rattle around as much. Then they can stick better to each other. The hard part to understand then is not so much why liquids get stickier and have a harder time flowing when they're cold but rather why they abruptly form real crystals of solid- ordered patterns of atoms or molecules.
These crystalline patterns are arrangements with lower energy than more general random arrangements. In other words, the molecules or atoms 'fit well' in these patterns. So as you cool the liquid, these are the patterns they fall into.
I still haven't said why there is abrupt freezing, not just a gradual tendency of the molecules to form more ordered patterns as they get colder. This is the interesting part. One molecule can't form any pattern by itself. Two molecules can't do much, because most of their surfaces won't even touch each other. The amount the energy per molecule can drop by forming a pattern actually gets bigger as more molecules get involved. So once a pattern starts to form, it grows into a real crystal, not just a small cluster of lined-up molecules.
One consequence is that when the liquid is cold enough so that it ought to freeze, it may take an extremely long time to freeze. The reason is that small clusters don't lose enough energy per molecule to make freezing 'pay'. Large clusters of the right type only happen very rarely by chance. Until one of them forms, the liquid sits in a 'supercooled' state.
(republished on 07/25/06)