Puppies and adult dogs bite for different reasons. Puppies use their mouths to explore the world and learn, and they bite on anything they can when they're teething to soothe pain. When older dogs bite, it may be because they weren't trained as puppies that biting is unacceptable, but it could also be a more serious issue. Teach your pet bite inhibition using a method appropriate for his age and circumstance.
Puppies in the Litter
Watch puppies in the litter and you'll see that they bite each other in play. Around 3 months of age, teething starts and puppy biting escalates -- and puppies learn from their littermates just how much biting is too much. When one bites another too hard, the bitten puppy yelps. Play stops, and the biting puppy finds out the fun is over. Before long, they'll be back to play, all the wiser.
Training the Puppy
It's normal for puppies to bite when they're playing, and it's normal for them to bite their handlers during this stage, too. Puppy teeth are razor-sharp, so puppy bites are painful. Teach a puppy early that biting human skin is not allowed. Begin training as soon as you bring your puppy home. You'll do the same thing his sibling did: As soon as your puppy bites, make a yelping sound or say "ouch" loudly. The sound you make must startle your puppy. You can do this up to three times in a 15-minute period. If he persists, you'll have to combine the yelp with a form of timeout.
The Timeout Method
If your puppy continues to bite, combine the yelp with a timeout: When you startle the puppy with a yelp, you could simply let your hand go limp and ignore the puppy for 10 to 20 seconds before resuming playing. Your puppy will come to understand that when he doesn't bite, play continues. You may need to get up and leave the room without the puppy for several minutes to teach the lesson, or even to place the puppy in a crate for a brief period. Your puppy will learn to be careful when he plays with his mouth. Continue to use this method to encourage gentler mouthing until it stops completely.
Adult dogs may bite simply because they not have learned that play biting is not allowed, but often when adult dogs bite it's because they're frustrated or fearful. Such dogs might bite because of related "fear aggression." Work with your vet and a dog behaviorist to help your pet overcome his fearfulness or frustrations. Similarly, a dog who's truly aggressive needs a professional's help. If your dog has never been a biter and has not shown a pattern of fearfulness bites you, he may be ill. A trip to the vet is in order.
Training Adult Dogs
To train a grown dog not to bite, you may have to employ methods other than those you'd use for a puppy. Redirect your adult dog's attention when he bites at you in play. Give him a chew bone or squeak a toy. Engage him in play that doesn't involve your hands. Throw a ball, or try using a soft tug-of-war toy. Teach your dog basic control commands such as "sit," "wait," or "leave it," and use them when your dog becomes mouthy. If redirecting him doesn't work, say "no" firmly when he bites; stand up and turn your back to him for a minute. This can be an effective timeout method for adult dogs.