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