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In my first yeshiva (which was a Chabad yeshiva), students would place loose hairs that had fallen from their beards into sforim. In my second yeshiva (which was a Litvak yeshiva), my rebbe made a point of blowing them out of books if the book was open and he saw one there. Where did this custom of placing hairs into sforim come from? How widespread is it? How widespread is opposition to it?