Go to Castle Town and head down the east road. Near the Medical Center is a bone which you can pick up. Walk down the street South of the Medical Center and keep going until you see a white dog. Go near the dog and it will start following you. Now go back down the street toward the Medical Center and throw the bone into a grass patch nearby. Sometimes the dog will bring you a rupee or heart instead. If the dog doesn't bring back anything it means that he didn't see you throw the bone. Also, you might have to leave Castle Town and do it again for the dog to fetch again.