Most normal people from Earth can tell the difference between a cat and a pig, without getting distracted by the fact that both happen to have four legs.

Aliens from the Alpha Centauri on the first visit to our planet, on the other hand, can be forgiven for mixing up the two. We will gently explain that many Earth lifeforms have four legs, and that we find it more significant and helpful to distinguish the Meow-meows and the Oink-oinks, so please, dude, quit insisting that the mud-wallowing fatty that we sacrifice for breakfast is a "cat."

But when the Centaurians start calling us humans "half-cats" based on our two legs, it's time to deport them out of the Milky Way.