Callie sits with her tennis ball. Come and get it.
The next time I walk by where she's sitting, she'll look at me with that look. It's the look that means "I want you to throw the ball." So I go and fetch the ball and throw it again. She chases it and then sits down with it. End of game. This will happen four or five times over the course of an hour or two while I putter around in the yard. At least it's not strenuous.