What Makes a Fish a Fish?

(Written by Anya Marasaka during a spell of boredom)

What makes a fish a fish?

There are many different types of fish, of all shapes and sizes.  But they are all, undoubtedly, fish.

The main defining characteristic of a fish is that it lives in the water, but is that enough to go by?

There are birds that cannot fly, and mammals that lay eggs.  And there are creatures which dwell in the water but are not fish.

If a fish were to dwell on land and breathe air like we do, would it still be able to classify itself as a fish?  If so, then why?

Where can the border between "fish" and "not fish" be found?

There are so many categories in the world that we take for granted, but that are impossible to explain simply without contradiction.

If, tomorrow, I were to meet a creature that naturally walked on legs and did not inject venom nor constrict, yet still called itself a snake...

...how willing would I be to believe it?