Science fun in the Big Freeze : SciencePunk

Supercooling
Everyone knows that water freezes at zero degrees celsius, but not many people know that ice crystals need a small imperfection or "seed" to grow from. This can be impurities in the water or microscopic imperfections in the container holding them. Without these, water will stay liquid well below freezing temperatures, in a supercool state. The tiniest disturbance will then cause all the liquid to freeze instantly! You can replicate this with a bottle of purified water (e.g. Dasani) left outside overnight.

Frozen Bubbles

via scienceblogs.com