Puppies and untrained dogs will get very excited over certain treats and will sometimes bite you while taking the treats from your hands.  This can make training a very unpleasant task.  To stop this behavior, when you are rewarding your puppy, hold the treat between your thumb and first finger in such a way that the puppy can’t get to it.  Hold your hand down so that the puppy can get to your hand without getting the treat and tell him easy.  Yank your hand away and loudly say ouch if he nips you.  Put it back down to him again and as soon as he licks or sniffs your hand without biting, move your thumb so that he can get the treat.  You want to encourage him to lick the treats out of your hands and not to pull them out with his teeth.  Be consistent when you are training to only release treats when your puppy is taking them properly.  It may take several training sessions for a habit of gentle treat taking to form and your puppy may forget and start most of his training sessions by grabbing with his teeth in the beginning.  Don’t give up.  Gentle treat taking will become a habit within a few days or at least within a week if you are consistent.