Freezing normally requires a starting point called a nucleation site. This site can be a speck of dust or a tiny scratch on a container. If a liquid is very pure and rests in a smooth container, crystals have nowhere to start growing.
The liquid gets trapped in a delicate state called metastability. It wants to freeze but cannot take the first step. If you shake the container or drop in a single crystal, the liquid freezes almost instantly.
This sudden freezing releases trapped heat into the surroundings. High altitude clouds contain supercooled water drops. These drops stay liquid well below freezing.
When airplanes fly through these clouds, the plane provides a surface for the drops to freeze. This creates dangerous ice on the wings. Students often think glass is a supercooled liquid because it lacks a crystal structure.
Modern chemistry actually classifies glass as an amorphous solid because its molecules do not flow. Understanding supercooling helps scientists preserve organs for transplant and prevent frost damage in crops.