Why does your puppy chew wood?

Dogs and cats will generally eat grass if they have an upset stomach. If it's a puppy the dog may eat bark or bushes because they are teething. Some dogs do it just for fun and there is really no nutritional value in it. My 7 month old pup will get pieces of bark and bring it in the house and flip it around and one time he even brought in a large branch off a tree! I must admit, for such a little guy and that big job of hauling that branch in I had a good laugh over it. Marcy His tummy hurts..he is not hungry( he or she)