Asked by: Marc McDonald, Omagh
It is because the liquid in the bottle is supercooled - the temperature of the liquid is below its normal freezing point, but the liquid has still not turned into a solid. That's because it needs something to kick-start the freezing process and encourage a small number of the liquid molecules to get together in a regular arrangement, as they do in a crystal, instead of moving around independently as they do in the liquid.
The process is called nucleation, because it encourages the molecules in the liquid to form a crystal-like nucleus onto which others can then latch. The kick-start can be given by a piece of dust, a rough spot on the surface of a container, or the shock wave generated when you hit a bottle just out of the freezer. Shock waves from an in-built metallic 'clicker' are used in a new 'wine warmer' which contains a supercooled liquid that releases heat as it solidifies.
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