Ice is the solid form of dihydrogen monoxide. It is in this state at or below 0 degrees Celsius (32 degrees Fahrenheit), at standard (1 Atmosphere) pressure.

When water is cooled to 0 degrees, further cooling begins the process of fusion. The latent heat of fusion is the amount of heat necessary to remove to complete the process.

The solid is formed by the crystallization of water molecules. The natural crystalline shape is hexagonal, however, extremes in pressure and temperature may cause other shapes.

Ice has a density of 917 kg/m3. This is notable especially because water, at 4 degress Celsius, has a density of 1000 kg/m3. Ice, being less dense than water, floats in the liquid - this is very exceptional, as most (if not all) other substances are denser when solid.

It is suspected that the lower density of ice has had substantial impact on evolution. After all, there are large amounts of life in the various bodies of water around the planet. Because of the physical properties, lakes and oceans would freeze from the top down, which in some ways would insulate water at lower depths from the temperature, making it less likely to complete freeze.