from most of the questions I read here, this one is one of the most childlike one, like a real 5 years old would ask.
so if I were to explain this to a five years old I would answer it like Because we don’t want to walk with leftover feces stuck in our butt hole / cheeks, it’s icky and smelly.
But tbh that’s why different cultures have different method of “wiping” after defecating, some use water, etc. the bottom-line is everyone just don’t want to smell bad and get rid of the leftover feces.
And similar to human other animals do they clean their butt when they feel the need for it
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