[Coda]

Of course, the Celsius scale isn't defined by the freezing point of water anymore. Now it's defined by setting the triple point of a very special form of distilled water (one that has a specific abundance of different isotopes of hydrogen and oxygen) as 0.01C and absolute zero as -273.15C. (The triple point of a substance is a temperature and pressure at which solid, liquid, and gas can coexist in equilibrium.) This means the boiling point of water at sea level is technically 99.9839C, not 100C.

The change was made because water's boiling point varies pretty dramatically with barometric pressure. If you pick up a pot of water 28 centimeters, its boiling point decreases by 0.001C!