A turbine is a rotary mechanical machine which effectively extracts energy from a fluid flow and converts it into useful work. A turbine is known as a turbo-machine. Early turbine examples are windmills and water-wheels. Turbines are of gas, water and steam and have a casing around the blades to contain and control working fluid. Almost all electrical power on Earth is produced with a turbine of some type. Most jet engines are dependent on turbines. Turbines are often part of a larger machine. Combustion turbines and steam turbines can be used for propulsion of ships. Aircraft engines can also use a turbine. Turbines can have very high power density.