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Most things get denser as they get colder. Water is one of the major and most important exceptions to this. Water is at its densest (without invoking the various exotic ice forms) at 4°C. It gets lighter as it gets colder.

Without this property there could have been no life in Earth. If water acted like almost all (perhaps all?) other liquids, its solid form would be denser than its liquid form and ice would sink to the bottom, meaning all bodies of water would freeze solid bottom to top in cold weather. The fact that water freezes only at the top is why life could form and why non-aqueous liquids are unlikely to allow life to form.