Wikipedia:POTD column/February 10, 2006

Liquid water is most dense at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the temperature drops to 0 °C. This is due to hydrogen bonds forming between the water molecules, which line up molecules less efficiently (in terms of volume) when water is frozen. The result is that ice floats on liquid water, an important factor in Earth's climate.

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