Krypton is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere. Krypton is extracted by distillation of air that has been cooled until it is a liquid. It is characterized by several sharp emission lines (spectral signatures), the strongest being green and yellow. Krypton is used commercially as a filling gas for energy-saving fluorescent lights. It is also used in some flash lamps used for high-speed photography. Unlike the lighter gases in its group, it is reactive enough to form some chemical compounds. For example, krypton will react with fluorine to form krypton fluoride, which is used in some lasers. Krypton has no known biological role. Krypton is also a fictional planet in the DC Comics universe, and the native world of the super-heroes Superman and, in some tellings, Supergirl, and Krypto the "super dog".