Ice

Ice is water frozen into a solid state. It melts at 32 degrees Fahrenheit or 0 degrees Celsius.

Unlike most liquids, water expands and becomes less dense when it turns into ice, so ice floats on water. This has the important consequence that rivers and ponds rarely freeze solid; a layer of ice forms on the surface but water remains below, enabling fish and plants to survive.

Astronomers routinely refer to other crystalline solids, with low melting points as ices, many astronomical objects have deposits of these substances. In particular, crystalline solids that look similar to water ice, are found on comets, almost all moons, almost all asteroids.

Exotic forms of water ice form in unusual environments, which have different densities, melting points, than ordinary water ice. Overhanging Kurt Vonnegut's novel Cat's Cradle is the danger posed to life on Earth of the discovery of a form of ice that is a solid at room temperature. Scientist try to make sure their samples never touch a natural body of water, as they fear if this happened every body of water on planet earth would freeze solid, as would every water-based organism.