Lola and especially JD get through tennis balls at an alarming rate. Left alone with a brand new tennis ball, he'll first have the skin off and then split it in half inside of 5 minutes. Which is why they have to be supervised during their ball play. We buy them cheap, used by the box full from e*ay sellers. I spotted a little card at the sports centre the other day "Used tennis balls 20p each" - that's half what we pay usually. The good people at the sports centre were delighted to unload a bag of them to me, and I got a bargain. I'll be back again next month for another tenner's worth.