An aspect about your body you took for granted?
I'm specifically talking here about something you didn't anticipate. Can you think of anything harmful that you'd been doing to your body over time, that you were completely unaware of? Or was there any habit that you'd been doing on a regular basis, that thought little of, but then one day realised that it had been having an effect on your body?
For me, it's my hips. I first noticed a clicking noise when I was 27, as I would turn into the foetal position when in bed. It was something I'd never have had anticipated. Physiotherapists tell me it's nothing to worry about, and that it's not a sign of ageing. But why has it started all of a sudden? I think that as I've gotten older, that DOMS seems something I've to be more weary of after exercise.
Another example would have been years of hunching over to see my fret board (as I played guitar), which I'm almost certain played a part in a trapezius problem I later got. Tinnitus is another. It came in my left ear about 2 years ago. 6 months later, I got it in the other ear. Doesn't bother me, but I mostly need to sleep with audio of "rain" or "waves on the beach". Hope my future wife won't mind that!
The more recently observed, and more bizarre issue, would be something I noticed that could be happening to me as I pick my nose. I was always aware of the puppet lines that form over time, near the mouth and nose, as we smile and laugh. Thankfully I don't really have these lines. But I've only realised recently, as I picked my nose while looking in the mirror, that as I shove my pinkie finger deep inside (as I do) and twist, that this causes a crease. The creases caused by this differ from the creases we get from smiling, in that they are nearer to the nose, rather than the mouth. So if I were to keep doing this, I might end up with puppet lines that are deeper towards the nose than in the areas lower down.
I know, most people just blow their nose, but I love that morning pick. There's quite a sensation when you feel the hardened snot shift inside your nose, and then come out almost like a mold. Then once the hardened snot is out, there can be huge amounts of fresher snot behind it.