In Munich in the days of the great theoretical physicist Arnold Sommerfeld (1868–1954), trolley cars were cooled in summer by two small fans set into their ceilings. When the trolley was in motion, air flowing over its top would spin the fans, pulling warm air out of the cars. One student noticed that although the motion of any given fan was fairly random—fans could turn either clockwise or counterclockwise—the two fans in a single car nearly always rotated in opposite directions. Why was this? Finally he brought the problem to Sommerfeld.

“That is easy to explain,” said Sommerfeld. “Air hits the fan at the front of the car first, giving it a random motion in one direction. But once the trolley begins to move, a vortex created by the first fan travels down the top of the car and sets the second fan moving in precisely the same direction.”

“But, Professor Sommerfeld,” the student protested, “what happens is in fact the opposite! The two fans nearly always rotate in different directions.”

“Ahhhh!” said Sommerfeld. “But of course that is even easier to explain.”

Devine and Cohen, Absolute Zero Gravity, p. 96.