Supercooling is when a liquid is below the freezing point, but no crystals form so it does not turn into ice. Once you disturb it the ice crystals form. The Coca-Cola company also briefly marketed special vending machines containing Sprite in the UK, and Coke in Singapore, which stored the bottles in a supercooled state so that they would turn to slush upon opening.
I have seen this happen with 1/2 drank bottles of soda in my garage. Pretty cool.
Youtube link to vids of supercooling.