What's something about you that very few people know, not because it's a secret, but just because it doesn't come up very often?
The first thing that came to mind is that I'm a trichotillomaniac, which is a compulsive hair-puller. Not figuratively, like "Republicans make me want to pull my hair out!" but quite literally. People who suffer from trichotillomania compulsively pull out their scalp hair, eyelashes, eyebrows, etc., generally due to stress, and most (80%) trichotillomaniacs are women or girls.
I have a pretty mild case, and I can generally control myself by sitting on my hands, when I even realize I'm doing it (or someone points it out to me). But there have been times in the past when my anxiety has been so high that I've given myself a small bald spot.
I used to be sort of embarrassed by it, but I outgrew giving any fucks. It's such a ridiculously strange habit that I now find it rather amusing.