By William Poundstone

Clever Hans was the world’s smartest horse. So it appeared, anyway. Hans and owner Wilhelm von Osten—a retired Berlin math teacher and phrenologist—traveled around Germany giving free demonstrations of the horse’s uncanny intelligence. Hans could understand German, spoken or written, and reply to questions by tapping a hoof. The answer had to be yes-or-no or numerical, indicated by the number of taps. Von Osten would ask the horse questions such as: “If the eighth day of the month comes on a Tuesday, what is the day following Friday?” The horse tapped its hoof 11 times.

Through this language of taps, the horse could tell time, spell, and do algebra. One contemporary report marveled: “He can distinguish between straw and felt hats, between canes and umbrellas.”

Crowds cheered on the demonstrations even as skeptics huffed that something must be amiss. The German board of education had psychologist Carl Stumpf organize a committee to investigate. The Hans Commission, as it was called, included the director of the Berlin zoo, a veterinarian and a circus manager. In 1904, this blue-ribbon panel reported that no trickery was involved.