When I was building and testing the sprite objects for Requires Mouse, I built this silly little toy.

Click on the canvas to grow a circle. Click a grown circle to make it shrink. If you click a circle while it's growing or shrinking, it will stop; the next time you click it, it will shrink or grow, respectively. If a circle shrinks to nothing, it's gone.

Surprisingly, you can actually draw some neat-looking stuff with this.

If you're interested, here's the source code.