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The multiple dropping tennis ball one is great. I force my dog to play fetch by bringing two of them into the front yard. He's a fetch asshole: He'll grab it and run off, trying to get you to chase him. So I throw one and when he comes back for the taunt I show him the other. He gets so upset that he doesn't have the one in my hand that he drops the other one. I throw it and he runs off chasing it. Pick up the one he just dropped, wash-rinse-repeat.