Learning how to moderate the force of a bite is very important for all dogs. Puppies naturally nip at each other while playing. If they bite too hard on their mother or littermate, the other dog will likely make a loud yelp sound, warning the puppy, “Hey, that hurt!” and the pupies learn not to bite so hard as a result.  If you rescued your pup or even if you purchased your pup from a breeder, it is possible they did not get enough practice before they came home with you.  It is even more possible that they just don’t know that the same rules apply with fingers, toes and clothing as it did with their littermates.