Why does superheating and supercooling occur?

Question

Why does superheating and supercooling occur?

Answer 1

Superheating:  This refers to a situation where a liquid (like water) is heated past the boiling point (100C for water).  It should turn into a gas at this point, but in certain conditions, it can be made hotter.  Specifically, this situation involves make sure that the surface area of the water is low, and that there aren't any available bubbles in the water for gas to form on.  As soon as the water is disturbed, it will erupt as a bunch of the water liquid turns into gas.

Supercooling:  The same process occurs, but this time, no crystals are allowed to form in the liquid.  Crystals tend to grow off of small seed crystals, so the lack of any seed crystals will allow the liquid to cool past its freezing point.

Basically, the phase change from liquid to gas or liquid to solid requires a starting point.  Liquid to gas requires a gas starting point for the phase change; liquid to solid requires a solid starting point for the phase change.  These spontaneously occur in normal situations, but in a lab, it is possible to avoid them and cause weird temperatures in a liquid.