Puppy Behavior

Question
I have a 18 to 20 week old German Shepherd mix.  He is neutered and up on his shots.  He hasn't bitten, but nip at me and other family members when excited during play.  If told "no" he stops.  What he will do is put his mouth over my forearm when I'm petting him.  He doesn't apply any pressure, but I still worry that as he gets older that might not be the case.  He doesn't appear aggressive or angry when he does it.  Just calm.  Is this okay or should I break this behavior?  Why is he doing it?

Answer
Hi,

I apologize for the delay, I went away to shows without checking the site for questions.  

This is normal, but not "OK".   This is how pups play with other pups.   So discourage him from mouthing, his teeth should never touch you.   Sounds like he is a very nice puppy.   Keep  up the good work.

Lori