What is supercooling and how do instant ice tricks work?

The answer

Supercooling occurs when a liquid is cooled below its freezing point without solidifying. Very pure water in a smooth container lacks the nucleation sites (tiny imperfections or particles) that ice crystals need to start forming. It can remain liquid down to about −48°C under certain conditions. When you disturb supercooled water by tapping the container or adding an ice crystal, nucleation cascades instantly through the liquid, and it freezes before your eyes in seconds. This same principle is used in some commercial hand warmers.