His disguise is very thorough, and from the outside he looks completely like the other species. However his bone structure is different, and he lives in fear of needing an x-ray and being discovered. Anyway, lots of stuff happens, and eventually he is with the ship's doctor, who has just taken an x-ray and thus has evidence of his deception.
It turns out that she has known all along (but it was okay, she didn't give him away). The reason she's known is because of his fingernails. At the base of each nail, there is a little half-moon, and she reveals that all humans have these moons, but aliens don't.
The point of my remembering this fragment of a story is that my fingernails didn't have moons on them. It was always a simple secret childhood pleasure of mine, that I wasn't like any other human being, with my alien moonless fingernails. I would frequently look at other people's fingernails, to see if they had moons. Everyone I checked always did -- some people don't have them on all of their fingernails, but invariably the thumbs at least would have them.
I would show people my freakishly moonless hands, and I would explain that as far as I knew, mine were the only ones like this. Sometimes I would even elaborate that all humans were supposed to have them. It added an air of truth to a common childhood fantasy, that of not belonging to the family or people you're surrounded by.
Well. The other day, I was looking at my fingernails for some reason, and I found, there on my right thumbnail, a little tiny moon! I picked at it for awhile, hoping it was just a discoloration on the nail or something, but it's remained where I found it. Only 1 of my 10 fingernails has one, but it doesn't seem to be going away. A moon, it just might be a real moon. Gently taunting me and all my childish pleasure in being inhumanly different. Darn it!
I wonder....what's it doing there, and where's it been all these years??