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Before mirrors were invented, almost no one had ever seen their own asshole
(startrek.website)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
reflection in water
....you mean the exact situation mention in the OP?
It would be common to see still water and your own reflection
Less common that someone would position themselves in such a way they can see their asshole clearly in it without disturbing the surface.
What do I know? Maybe they made little foot jetties just so they could stand above the water and gaze wistfully in into their own rectums on still days?
Narcassus