What to do with your old tennis balls

I’m sure, like most tennis players, you can only use so many tennis balls.  You can only visit so many dog parks. And, they are only good for practicing with for a short time.  So what else do you do with them?  I once was reading a “Wanted” forum that someone said they needed tennis balls in any condition.  I replied.  I asked how many they wanted.  She responded that they needed approximately 800.  What did she need them for I asked (as my curiosity got the better of me).  She said that she was the principal of a little school in rural North Carolina, and they had finally refinished the floors, and wanted to put tennis balls on all the chairs to keep from scratching up the new floors.  I said I’d have them within 2 weeks.  She said a man named Robert Lee would pick them up.  Finally got over 1000 balls, and sure enough, he arrived one Saturday morning to pick them up.  Just never know.  Other uses that need them:  Physical Therapists — they put them on the legs of walkers for the elder.  Marinas — people put them over there trailer hitch ball when not being used.  Animal Shelters — never seem to have enough things for the animals to play with.  If you have other ways to use tennis balls, please reply and let us know.