To drive a wheel (that is, to make it turn), something must put a torque (twisting force) on it. For machines made by people, this torque is usually made by a motor. There are many kinds of motors, such as electric motors, turbines, and steam engines. Torque can also come from a person, as with a bicycle. In animals, movement is usually caused by skeletal muscles. These muscles get energy by metabolizing (breaking down) the nutrients in food. Skeletal muscles are attached with connective tissue to the bones they move, so they would not be useful for driving a wheel directly. Also, animals use their muscles in cycles - joints move in one direction, then the other, instead of moving in one direction constantly. Large animals cannot gain speed very quickly, because it is more difficult for muscles to pull against a large inertia (the resistance of a heavy object to movement).