This is a fixed point on the Celsius temperature scale ... near Hamburg has now succeeded in precisely measuring the nucleation of supercooled liquids. The experiments took place in a vacuum ...

Inside is a metal clicker, once the pack cools to room temperature, crystallization of sodium acetate (an exothermic process) is triggered by the clicker, which gives the sodium acetate a nucleation ...

so that the critical conditions for nucleation of the liquid are reached first, unless working with very low vapour pressures far below the melting point. This is also in agreement with observation.