Why Ice Floats

Molecules crowded close together in the liquid, locked into a roomy open frame in the solid, and an ice lid sitting over a lake with everything alive going on underneath it.

Before you start. All you need is this: whether something floats depends on how heavy it is for its size, not its shape - a coin sinks, a hollow plastic duck the same amount of plastic floats. Nothing else is assumed.

The idea in 30 seconds

Whether something floats is about how heavy it is for its size, not its shape - Galileo won that argument in 1612, using ice as his star example.
Nearly every solid is heavier than its own liquid; melt and refreeze butter, wax, almost anything, and the solid sinks in its own puddle.
Ice is the exception: freezing spreads its molecules into a roomy open frame instead of packing them tighter, because the tug between molecules only holds along four fixed directions.
More room between the same molecules means less weight for the same size, which is the entire reason ice floats.
Liquid water is heaviest a few degrees above freezing, not right at it, because a second effect - early frame-building - starts winning the cooling fight around four degrees.
That four-degree turnaround is what stops a lake’s autumn stirring before it reaches freezing, so an ice lid can form on top with a livable, unfrozen bottom underneath.

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