Self-HelpIt’s the Mechanical Sort, Not the Psychological, and Engineers Can Really Use.

As a child you probably learned, to your astonishment, that you could carry two heavy pails of water, one on either side, more easily than one. Or you might have delighted in those Chinese finger traps that finger-cuffed you to a friend no matter how hard you pulled. These are familiar examples of the often unfamiliar design principle of self-help. It's an odd name, self-help, for a principle of mechanical design. In popular psychology, self-help is all about solving personal problems without professional intervention. In mechanical design, it means letting the design deal with a problem from within, rather than intervening with more structure or force . Common examples that show the principle are as varied as scissors, broom holders, lift bridges, O-rings, building elevators, pressurized airliner doors, and knots.