Content Warning: This section includes discussions of animal euthanasia.

Riley’s tennis ball symbolizes William’s attachment to Riley. Not long after Riley is taken away by the animal control officers, William finds his tennis ball in the closet. The ball is one of the only tangible pieces of evidence that William has to remind him of his happy days with Riley. Even though the tennis ball is old, chewed, and still wet with spittle, William treasures it. It reminds him of how Riley would “even sleep sometimes with this ball in his mouth” (83). The tennis ball makes William feel the weight of his responsibility toward Riley. He worries about what will happen if he cannot successfully campaign to save Riley’s life. 

At the end of the novel, after William has succeeded in his mission of saving Riley from being euthanized, he says a final goodbye to Riley at the airport. He gives Riley the tennis ball back, which indicates his willingness to move on from everything that has happened. He is finally able to be at peace knowing that Riley will continue to have a happy life, even if he has to move far away.