
This is normal, though unwanted behaviour. Puppies don't have hands to explore with so they have to use their mouths. They just have to learn to do it gently! If you do a search (top right hand corner of the screen) and type in "Puppy mouthing" or "Puppy biting" you will find loads of previous threads on the subject.
I would recommend you buy a copy of "The Perfect Puppy" by Gwen Bailey. It is invaluable for all puppy owners!
:)