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For example, although the sight of ice cubes floating in a glass of ice water is commonplace, such behaviour is unusual for chemical entities

For almost every other compound, the solid state is denser than the liquid state; thus, the solid would sink to the bottom of the liquid. 

The fact that ice floats on water is exceedingly important in the natural world, because the ice that forms on ponds and lakes in cold areas of the world acts as an insulating barrier that protects the aquatic life below.

If ice were denser than liquid water, ice forming on a pond would sink, thereby exposing more water to the cold temperature.

Thus, the pond would eventually freeze throughout, killing all the life-forms present.

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