A puppy, like a human baby, explores his world with his mouth. Plus, the puppy's mouth is his only way to explore the world because he does not have hands to pick objects up with. If he finds something new, he will immediately put it in his mouth to see what happens. Your hands, feet and belongings are not exempt in his explorations and will be subjected to his bites. This behavior will last through the first three or four months of the puppy's life. Even though the biting is normal, do not allow your puppy to bite you as this will teach him it is an okay behavior.
Around four months of age a puppy begins to get her adult teeth. As a puppy's deciduous (baby) teeth fall out and her adult teeth grow in, she will start teething. To relieve the discomfort of her adult teeth erupting, she will bite and chew on anything she finds, including your hands. A puppy will have all of her adult teeth by the time she is eight months old. In the meantime, if the puppy is redirected toward appropriate chew toys such as rawhides and bones (nylon, rubber or real) she will learn what is and is not okay to teeth on.
A puppy learns social skills through play, and part of his playing involves biting. A puppy will bite his litter mates, his mother and any other creature he encounters in play, and the reaction of his playmate teaches him how to control his bite. When the puppy bites his playmate or mother, they react with a yelp and the mother may even correct the puppy by growling or snapping. If the puppy's biting behavior continues, playmates will stop playing with him and his mother will give him a more stern correction. Eventually, a puppy should learn through these play-fights how to soften his bite thereby developing bite inhibition. If a puppy is removed from his mother and his litter too soon, he may miss out on this valuable lesson. A puppy who does not learn bite inhibition will be a more dangerous dog. Dogs sometimes give warning bites to other animals as well as people, and if he has not learned bite inhibition, he may bite hard with the first bite and injure whomever he is giving his warning bite to. A puppy who does not develop bite inhibition by biting will also play rougher than a puppy who has.
Sometimes through inappropriate play you can teach your puppy to bite more. In "The Power of Positive Dog Training," Pat Miller warns you to never allow your puppy to bite you in a game of tug. She instructs you to end the game and put away the toy should this occur. Wrestling is another game in which you might teach your puppy it is okay to bite. If your puppy bites you while you are playing, and you allow it, your puppy will learn that biting is allowed.