Liquefaction is a term used in materials sciences to refer to any process which either generates a liquid from a solid or a gas, or generates a non-liquid phase which behaves in accordance with fluid dynamics. Liquefaction occurs both as part of natural processes, and in man-made processes used in science and commerce. For example, "[a] major commercial application of liquefaction is the liquefaction of air to allow separation of the constituents, such as oxygen, nitrogen, and the noble gases", while another application is the conversion of solid coal into a liquid form usable as a substitute for liquid fuels.