Physics
Why does ice float?
Water is one of very few substances that is less dense as a solid than as a liquid. If ice sank, lakes and seas would freeze from the bottom up.
Source: Standard chemistry; gallium and bismuth share the trait.
Context
Because water is one of very few substances that is less dense as a solid than as a liquid. Hydrogen bonds hold the molecules in an open lattice when they freeze.
If ice sank, lakes and seas would freeze from the bottom up and could stay frozen. Instead the ice floats and insulates the water beneath. Gallium and bismuth share the trait, but they are not what fish live in.
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