The Walgreens on my campus always used to sell red cups and ping pong balls together like this too. They also would sell Jell-O mix right next to the mini Dixie Cups.
I showed this picture to DH who said "Nah, it's not what you think, it's for a science fair project". Yeah, right!
__________________ "If you want to criticize my methods, fine. But you can keep your snide remarks to yourself. And while you're at it, don't criticize my methods." Rupert Giles, BtVS