Okay, we know the standard uses. Fido, bottom of school chairs and desks, feet of walkers. here’s some more uses for old tennis balls. This list is from a blog called “Life Hackery” (Useful, Unusual and (Sometimes) Ironic Tips and Tricks to Hack Your Life into Shape)
3) Hang one on string from the garage roof to help you park without running into things. When it touches the windshield or rear window (depending on which way the car is pointing), you know it’s time to stop. If you’ve got a lot of time on your hands, make it into a funny character.
27) Put tennis balls on the tops of poles to mark the edges of your driveway or drainage ditch. The bright yellow balls will be visible in the dark and help you avoid driving into the ditch or over the grass in the dark.
See all 50 – HERE