Poison
A rib cage hanging in the window of a shop indicates they sell poison. It's a universal sign, kinda like barber poles or those crosses on pharmacies.
どく
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Ask any poison vendor: the middle of a Sudoku (どく) puzzle is the best time to poison someone. They'll be concentrating so hard on their Sudoku puzzle that they won't even notice the poison begin to set in… until it's too late.