A machine cannot create energy. What it does is to produce force and control the direction and motion of that force. A machine's work output can never exceed the amount of energy put into it. Its function is to transform one form of energy, such as electrical energy, and pass it along as mechanical energy. Some machines are known as prime movers because they change energy directly into mechanical motion. These include diesel engines, stream and water turbines and windmills, among others. As an example of how a prime mover operates, the energy of falling water rushing through the wheel of a turbine produces rotary motion. This direct motion of the wheel can be used to turn a generator that produces electricity. Other machines, such as a generator, a water pump, or a harvesting machine, are run by prime movers. These machines only control or produce certain forces and motions. A machine's ability to do work is measured by two factors: efficiency and mechanical advantage. The efficiency of a machine is the ratio between the energy it supplies and the energy put into it. Machines that transmit only mechanical energy may have an efficiency of nearly 100 percent. But some machines have an efficiency as low as 5 percent. No machine can operate with 100 percent efficiency because the friction of its parts always uses up some of the energy that is being supplied to the machine. All machines produce some friction. For this reason, a perpetual motion machine is impossible. A simple lever is a good example of a machine that has a high efficiency. The work it puts out is almost equal to the energy it receives, because the energy used up by friction is quite small. On the other hand, an automobile engine has an efficiency of only about 25 percent, because much of the energy supplied by the fuel is lost in form of heat that escapes into the surrounding air.