The brisker rav used to give his sefarim to bar mitzvah boys and hide a $50 bill in the pages (in those days a lot of money). A few months later he would visit the boy’s house ostensibly to look at the shelves of new sefarim he had amassed. He would then open the sefer he had given and take back the $50. Never once had the boy actually opened the sefer. Moral of the story?
