The recent below-freezing temperatures, while not the most pleasant to experience, do give us opportunities to explore some pretty interesting physics. One of my favorites is the phenomenon of supercooling a liquid. 

We're used to the idea of dropping the temperature of water beneath its freezing point and it becoming ice. Normal routine so far. But if the water is especially pure then it can be cooled far lower while still maintaining is liquidy state. This is because phase transitions, when a material completely changes character, need a starting place to get the party going.

This starting point has a name, of course: the nucleation point. A tiny imperfection, like a mineral or a speck of dust, allows the water molecules to cling to something and start lining up in the familiar regimented order of a solid. Without that impurity water can stay as water.

But once the impurity is introduced the phase transition takes place as rapidly as it can (because the water is super cold) and before you know it you have a block of ice.