Put a small amount of dry sand into the pendulum. Move it gently to the side and let it swing freely. Watch the picture drawn by the sand.
How it works
If the pendulum only swings in one plane (there are two possibilities), then the sand draws a straight line. In other cases, the oscillation happens in two planes at the same time. The drawings are determined by the ratio of component vibration frequencies and the oscillation starting point; the figures obtained are called Lissajous (pronounced lee-suh-ZHOO) or Bowditch curves. To get a different drawing, you should change the vibration frequency ratio. This can be done by moving the slide along the cord. If this ratio is a rational number, the produced drawing is a closed curve.