At low temperatures, much of the matter that surrounds us is in its solid state. Water is nearly unique among all substances in that, in its solid form, its density is lower than that of its liquid form. Not to put too fine a point on it, ice floats. It is because of this peculiarity that life can exist. Imagine an earth where ice sinks. The oceans would have a bottom layer of ice, in some places kilometers thick.

Ice is intricately ordered, three-dimensional, and crystalline. It has the lowest internal energy of the "common" states of matter.