> When I was a boy in England long ago, people who traveled on trains with dogs had to pay for a dog ticket. The question arose whether I needed to buy a dog ticket when I was traveling with a tortoise. The conductor on the train gave me the answer: "Cats is dogs and rabbits is dogs but tortoises is insects and travel free according."
https://www.goodreads.com/quotes/7236984-when-i-was-a-boy-in...
Anyway, turtles float. Spaceship re-entry vehicles float. Coincidence? There are no sharp edges on turtle shells, showing they've evolved to produce a detached shock front. I think that makes a convincing case that turtles are from space. Nothing else looks like them[0] on this planet, further buttressing my argument.
[0] Tortoises don't count.
Turns out ontologies are confusing and probably can be some of complete, consistent, coherent or correspond to common sense but not all of the above. Whenever these types of debates come up (eg "Is a tomato a fruit or a vegetable?" or indeed any "Is x a type of y?") I'm reminded of Borges[1].
[1] https://en.wikipedia.org/wiki/Celestial_Emporium_of_Benevole...