Harmonograph

A new window should open with a randomly generated figure like the one above. It will draw a green figure. I recommend maximizing the window. Click within it will generate a new figure, but this make take some time depending on the speed of your computer. Don't keep clicking when it doesn't do much because it will take a long time for it to catch up and respond to all of those clicks.

Here is a link to a page with instruction on building a real harmonograph.