Using a torsional oscillator, it will be shown how its amplitude can be increased without external torques, merely by correctly timed variations of the moment of inertia, which requires work. This is the principle of setting a swing in motion, or of swinging on the high bar. The oscillator used is a rotating stool with a helical spring. The experimenter on the stool changes the moment of inertia by alternating his position from sitting to prone.