Taxonomy, or the science of classifying organisms, is one of those fields that sounds easy only because you couldn't possibly understand how complicated it is, like candle-naming or Beyonceing. How hard can it be to tell if something is a cat or a dog? It's probably all you do whenever you're outside. You see a dog walking toward you, you announce to your friends, "DOG!" There, you just taxonomied.
But that's just the adorably primitive pattern-recognition functions of your dumb caveman brain. Your instinct is to look at, say, any animal that has gills and scales and tastes deliciously oceany, call it a "fish," and never question it, but taxonomists know that gilly bastard could be anything. For example, the tuna has been found to be more closely related to seahorses than many other kinds of supposed "fish." An ancient species called coelacanth is actually more closely related to humans than the salmon it resembles.
When it's possible for evolution to take a shorter path to Megan Fox than to a carp, it doesn't really make much sense to call that animal a fish. And that's the case for, like, every fish.
That's why, following the advent of modern genetics that revealed these oddities, some scientists have started advocating for discarding "fish" as a useful category of animal. If you wanna call something a fish, taxonomically speaking, you've gotta call everything more closely related to it than other supposed fish -- which could be anything from a manatee to a mouse -- a fish, too. It definitely throws a moral wrench into future trips to Red Lobster, because technically, you are a fish, but screw it. Embrace your gulpy existential crisis.
Top image: Priya Deonarain/Flickr