DS is 16 months and for the last few weeks has been gnawing on anything metal - belt buckles, spiral notebooks, you name it. He'll chew on other stuff too (toys mostly) but it's the metal I'm worried about because of his teeth. Is this normal? Is he just teething? (He never went for any of the "official" teething things.) Or could he have some sort of weird mineral deficiency? He doesn't do it all the time but enough that I've noticed and am posting....