When a tennis ball is dropped through the top of the mystery tree, it falls randomly into one of the three "root" chambers. Each root has a unique release mechanism in which the dog must figure out how to successfully retrieve the tennis ball. Each time you drop the ball into the toy, you provide a new mystery for your dog! For more exercise, throw the ball and play fetch after each successful retrieval. For more advanced play, teach your dog how to drop the ball inside the top chamber on its own. One tennis ball is included. For dogs less motivated by tennis balls, you can provide a treat upon success.