The drinking duck, or bird, is, in principle, a heat engine composed of an empty sphere and a bulb filled with a volatile liquid (usually methylene chloride), connected by a pipe that rotates around a horizontal axis. A difference in pressure develops because the wet duck's head cools by evaporation, while the liquid inside gets warmer. The duck starts to bounce as its center of gravity rises and falls. The duck leans forward when the liquid goes down, restoring the pressure equilibrium. The beak moistens and the water evaporates, letting the head rise, restarting the cycle. The device thus simulates a perpetual motion machine.