Water

Water, chemical name dihydrogen monoxide, is a common chemical substance that is essential to all known forms of life. In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor. It's chemical make-up is a covalent bond of two hydrogen atoms and one oxygen. The covalent bond is strong, but the energy released from breaking this bond is low and hard to capture.

Appearances in the Halo Universe

 * Water is seen in plain sight in six of the ten levels of Halo: Combat Evolved, nine of Halo 2's levels, and almost all of Halo 3's levels, in some form or another (excluding water vapor).
 * Frankie is a self-confessed fan of video game water.