A physicist was asked to investigate the cause for the periodic bumping in a running railway train. First he checked the locomotive. But the locomotive moved very smoothly. Therefore the physicist disregarded the locomotive as cause for the bumping, and he could concentrate on the train of cars behind the locomotive.


He walked from one car to the next, and he realized that the regular bumping noise was the same in each car, except that it was delayed from car to car by a constant time delay. He concluded that it is sufficient to investigate a single car. Each car was composed out of two parts: the lower and the upper structure. Since the upper structure did not contain any moving parts, they could not cause any periodic bumping noise.


Now the physicist was left with the lower structure of the railway car. This structure contained as only moving parts the axle where the wheels were mounted on. But the maintenance officer confirmed that all axles have been properly greased thus avoiding any noise with the running wheels.


So the physicist could reduce his investigation to the only moving parts left, namely the wheels. Since all these wheels were perfectly circular he recalled the formula for the circle's area: pir. Next the physicist checked the first argument in this famous formula: He recalled that pi is absolutely constant and has a value of about 3.14159265358979...


So he checked for the radius r: but the thermal expansibility of the wheels could be neglected, particularly because of the different time constant of possible thermal movements. Finally he saw the square in the formula and detected the solution.


Of course, he said, this square explains everything: a square never could roll smoothly on a plane surface.