Most liquids contract by about 10% when chilled. Water does too, but only down to a point. Once it is about to freeze, something entirely unpredictable happens — it expands! By the time water solidifies it has almost a tenth more volume than it did as a liquid.
Who cares?
Well, if water did not expand as it began to freeze then ice itself would sink, and the lakes and oceans would freeze up over time. That is because the floating ice holds the heat of the water beneath it, creating a kind-of “lid.”
So because of its unique properties, we have a watery planet, not a frozen one.
