Shlomo Klein worked for the security department of a large retail store, and his duties included responding to fire and burglar alarms. A side door of the building was wired with a security alarm, because it was not supposed to be used by customers.
Nevertheless customers always found the convenience of the exit tempting. Even a sign with large red letters, warning "Alarm will sound if opened," failed to deter people from using it. One day, after attending to a number of false alarms, Shlomo got fed up and placed a small handmade sign on the door that totally eliminated the problem: "Wet paint."