When you first get a dog from the adoption center they are children. Once you keep petting them and takeing care of them the hearts turn pink. When all your hearts are filled up with pink and arn't turning grey that means that your dog is an adult. When there hearts are all filled up they are ready to find a mate and have a puppy. The only way you can get a dog that is a puppy is when your adult dog has one. Then keep taking good care of the puppy and when his hearts are filled up he will turn into a child. Then keep good care of him and when his hearts are all filled he will turn into an adult and the hearts will stay pink.