First, nipping and mouthing is completely normal canine behaviour. However, it can develop into a serious problem if reinforced and allowed to continue into adulthood. An adult dog’s mouthing is a lot stronger than a puppy's, so even when it's just part of normal canine play style, it can still cause damage and pain to humans. After all, there's no such thing as puppy biting. It's just a puppy that's doing it at the time but if allowed, you'll soon have an adult dog doing the same thing... With a lot more strength!