The cat watches the dog play with a full sized tennis ball and the cat has it's own mini tennis ball. They like to roll around with it and chew on it like the dog does.
Walter grasps the ball and rolls around until he is satisfied.
His back feet get in on the action and uses them to grasp the ball better.
Sometimes he flings it while wildly playing with it hurtling it across the room.
Well I guess he learns from the dog. They are good friends of course.