A motor driven turntable has a vertical post mounted near the rim. Collimated light projects the shadow of the moving post on a rear mounted panel at the back of the exhibit. The velocity of the shadow is sinusoidal. A pendulum swings along the rear of this exhibit with a small circular screen as its bob. The visitor is instructed to start the pendulum in sync with the moving shadow of the post. With a little practice, the pendulum can be started at just the right moment to keep the post's shadow on the pendulum's moving screen.