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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.
I wonder if a human back then used the reflection from water at a lake or pond to see what their asshole looked like.
Remember that the first mirrors were made from metal and polished volcanic glass.
Which means mirrors or mirror substitutes were always available to us even before we made metal tools as long as we could source obsidian. Granted you'd have to actually live near an active volcano for the latter.
However you aren't wrong! A cheap way to make "mirrors" in the old time is to just take a flat bowl with mud lining the bottom and have water on top. This was used when volcanic rock of polished stone/metal was not readily available!