Draw your own sine wave!
This contraption is analogous to a compass. A compass is a simple device for drawing a circle. This device lays your circle out along an additional dimension. A sine wave is drawn as the model is moved along a piece of paper. Alternatively, the paper could be moved and the model is held fixed. As the wheels turn, their vertical circular motion is transformed by the worm into the horizontal circular motion of the white rotors. This moves the pencil (represented above by the vertical cross axel) back and forth.
The concept is illustrated in this animated gif on Wikipedia (http://en.wikipedia.org/wiki/File:Sin_drawing_process.gif ). Beyond it's intrinsic utility as a sine wave drawing device, a greater understanding of wave motion can be obtained by the physical act of building this tangible model.