Humans are weird
Just imagine an alien’s reactions to getting a tattoo. I’m picturing the conversation going something like:
“Wow human-Sandy, that is a lovely painting on your arm. How long will it last?
“Oh, it’s permanent. It’s a tattoo.”
“What’s a tattoo?”
“Well it’s kind of like painting on your skin, but we actually inject the ink into us with needles so it’s more permanent.”
“Oh. That’s… nice.” And as he walks away he’s just muttering about how humans drink poison that ruins their liver, and inject ink into themselves “for fun” and none of his crewmates believe him because no species could be THAT self destructive, not even humans!
Of course later, Sandy tells them all about it, and once again they are left completely confused by the entirety of human existence.