I buy tennis balls bulk but not why you would think. The dogs LOVE them. They chase them, slobber on them, hide them and chew them. One time, Jenny had one she had chewed a big hole in. She went in to take a drink out of the toilet an dropped it in. It sunk. It ended up blockin up the toilet. We finally got it out but with great pain and agony.
We seem to always have 4-6 balls laying around. But look how these two are so happy with the simple tennis balls.