If you watch a litter of puppies playing, you will notice that they spend much of their time biting and grabbing each other with their mouths. This is normal puppy behavior. When you take a puppy from the litter and into your home, the puppy  will play bite and mouth you. This is normal behavior, but needs to be modified.

The first thing to teach your new puppy is that human flesh is much more sensitive that of other puppies and that it really hurts when they bite us. This is called bite inhibition. A puppy has very sharp teeth and a weak jaw. This means that the puppy can cause discomfort when mouthing or puppy biting you, but cannot cause severe damage. An adult dog has duller teeth and a powerful jaw. This means that an adult dog can cause significant damage when biting.

ANY DOG WILL BITE GIVEN THE RIGHT OR WRONG CIRCUMSTANCES !
If a small child falls on your adult dog and sticks a finger in the dog's eye, you should not be surprised if the dog bites. If you do a good job teaching your puppy bite inhibition, you should get a grab and release without damage. If you do not, you may get a hard bite with significant damage.

BITE INHIBITION
It is simple to teach a puppy bite inhibition. Every time the puppy touches you with its teeth, say "OUCH!" in a harsh tone of voice. This will probably not stop the puppy from mouthing, but over time should result in softer and gentler puppy biting.

COMMANDS
The commands necessary to teach a puppy NOT to mouth, are easy and fun. Hold a small handful of the puppy's dry food, say "take it" in a sweet tone of voice, and give the puppy one piece of food. Then close the rest of the food in your hand and say "off" in that same sweet tone of voice. When the puppy has not touched your hand for 3 to 5 seconds, say "take it" and give the puppy one piece of food. We are teaching the puppy that "off" means not to touch. You should do this with the puppy before every meal for at least 5 minutes.

After a couple of weeks of the above training, here is how you are going to handle puppy biting or mouthing:

A.  Unexpected mouthing (you don't know the puppy is going to mouth until you feel the puppy's teeth):
 say  "OUCH!"

B.  Expected mouthing (you see the puppy getting ready to mouth you):
 You say "OFF"  before the puppy can mouth you.

C.  The puppy is mouthing you because of a desire to play.
 You have to answer the question, "Do I have time to play  with the puppy now?" If you do, then do "sit", "down",  "stand" or other positive 'lure and reward' training FIRST, then play.

If the answer is "No, I don't have time for the puppy, right now." Then you need to do a time out (crate the puppy), so the puppy can't continue to mouth you. Time outs are much more humane than yelling at a puppy.