The idea that energy can not be created or destroyed is the law of what

b. lithosphere, asthenosphere, mesosphere   is the answer most likely

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.