A popular puzzle, invented in 1878 by Noah Chapman. It is a set of identical square knuckles with printed numbers enclosed in a square box. The length of the side of the box is four times the length of the side of the knuckles for a set of 15 elements; accordingly, one square box remains empty in the box. The goal of the game is to move the knuckles around the box, to arrange them by numbers, preferably making as few movements as possible.