Density

What substance, other than water, is less dense as a solid than as a liquid?
Only bismuth and water share this characteristic. Density (the mass per unit volume or mass/volume) refers to how compact or crowded a substance is. For instance, the density of water is 1 g/cm3 (gram per cubic centimeter) or 1 kg/l (kilogram per liter); the density of a rock is 3.3 g/cm3; pure iron is 7.9 g/cm3; and the Earth (as a whole) is 5.5 g/cm3 (average). Water as a solid (i.e., ice) floats, which is a good thing; otherwise, ice would sink to the bottom of every lake or stream.
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