A playful article by a former LIDYL thesis student: the Bozo-Bozo
The Bozo-Bozo is an old puzzle, probably invented by the North American Indians. It’s made of a notched stick with a nail at the end, which supports a freely rotating propeller. When the notches on the stick are rubbed, the propeller starts to turn. The article shows that the secret lies in the way you hold the stick that rubs the notched stick: by pressing on either side of the stick, you disrupt the trajectory of the nail and can thus make it turn in a controllable direction.