Asking for a friend.
Michelle Rial / BuzzFeed
GOOD. QUESTION. My friend has been wondering same.
Well, it's called the the Mariko Aoki phenomenon, and according to the Wikipedia page, it could be caused by: "the smell of paper or ink having a laxative effect, the association with reading on the toilet at home, and the posture of browsing making bowel movement easier."
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There's A Phenomenon That Explains Why You Poop At Bookstores