How do you play fetch with a dog that won't give the ball back?

Get more balls!

I have finally figured Cotton out. If I have a ball in my hand ready to throw when he returns with the old ball, he will drop it at my feet and chase the new one. This was working pretty well, I even had a few extra balls for when Tessie sweeps in and steals the dropped ball the moment it hits the ground. Then Cotton was on to me. He started keeping the first ball in his mouth until he got to the other one. Okay, more balls and more changing sides of the kennel. But Cotton has come up with something that puts the whole game in jeopardy, he keeps taking the balls into his run with him. I didn't realize that he had pretty much cleaned out the playyard of toys until I had to empty both his water dish and food dish of tennis balls just to feed him.