An organ represents a higher level of structure than the tissue composing it and performs functions that the tissues cannot perform alone. this is an example of the principle of

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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.