A treadwheel, or treadmill, is a form of engine typically powered by humans. It may resemble a water wheel in appearance, and can be worked either by a human treading paddles set into its circumference (treadmill), or by a human or animal standing inside it (treadwheel). These devices are no longer used for power or punishment, and the term "treadmill" has come to mean an exercise machine for running or walking in place.