Below are the first four prototypes of a machine designed by elves to sort presents by weight. Each machine sorts four presents at a time. The four presents are placed in the top, and then fall through the slides. Where two presents meet at a crossing, the lighter present goes to the left, and the heavier one goes to the right. This is repeated until all four presents are at the bottom.

Four elves, Fredi (39kg), Oswald (34kg), Iphis (28kg), and Esmeralda (21kg) are selected to simulate the presents in test runs. Which one of the four machines sorts the elves correctly for every possible order in which the elves can step into the four slides?

To make things easier let’s call the elves from lightest to heaviest A, B, C and D, and call the positions 1, 2 3 and 4. The question asks us to find the machine that always results in the order A, B, C, D in positions 1, 2, 3 and 4.

80 percent of German kids got the right answer?!