Back when I used to smoke a lot of drugs, and come home at night in a less than sober state, I would always worry about forgetting to lock my front door and therefor have someone break into my house... and I would have to admit it was my inebriated state that led to the doors being left wide open. We have the turn-lock style ones on the door, and with the two bolts holding it on (eyes) it looks sort of like a duck when the lock is closed properly. Anyways, I would look at the locks before going up to bed and say "Duck means locked." to myself to affirm the doors were indeed locked. I no longer do drugs but I still find myself saying "Duck means locked..." absentmindedly when I check the doors before going to bed.