A month or two after I wrote the Stretchy Ball program, I tried making the ball slightly transparent. The result: a clear jelly effect. I thought it looked cool, so I saved it as a separate program and named it "Lens Ball".
Soon another user on Khan Academy suggested making the ball jiggle whenever it was clicked. That sounded good, but I thought it would be more realistic if it jiggled no matter WHAT you did. I modified the program accordingly, and once I set it up to slide down the screen, the effect was complete. Here it is - just click and stretch:
