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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.
It's funny you say that, because I was just watching a video of great apes looking at themselves in a mirror that scientists set up in the jungle. Once they got over the fear and realized they were seeing themselves, one of the things multiple apes did was look at their assholes.
They tried to look at every part of their body. Yes, this includes the asshole.