Watch this! A video shows water in a bottle freezing instantly just by hitting it on a surface.
Why does this happen?
It is because the water in the bottle is supercooled. A supercooled liquid is one in which the temperature is below its normal freezing point, but the liquid has not solidified. This is because the liquid needs something to initiate the freezing process - something to encourage a number of liquid molecules to get together in a regular pattern, which they do in a crystal rather than moving freely in a liquid.
The process water undergoes to become crystals, or ice, is called nucleation. Nucelation occurs when molecules in the water clump together, forming a crystal-like nucleus. Once formed, other molecules can latch on to the nucleus.
Several actions can trigger the nucelation process, including something as small as a piece of dust that the molecules can adhere to, or a rough spot on the surface of the bottle. Another trigger can be the shock wave generated by hitting a bottle just out of the freezer against a table.