If I put a bottle of water in the freezer it will not appear to be frozen until I hit it then the ice appears. Why is this?
Super cooling. If you keep the water very pure and perfectly still, you can keep ice crystals from forming a few degrees below freezing. If you introduce a foreign particle into the liquid, or create an agitation (whacking the bugger) ice crystals will form at this point the the chain reaction will freeze the rest of the container.