What horse became famous for seeming to solve arithmetic by tapping a hoof?

Clever Hans was the horse that became famous for seeming to solve arithmetic by tapping a hoof.

In early twentieth-century Germany, owner Wilhelm von Osten presented Hans as an animal able to add, subtract, multiply, tell time, and answer calendar questions. Von Osten would ask a question, and Hans tapped his hoof until reaching the apparent answer. The performances attracted widespread attention and helped fuel public interest in animal intelligence.

A formal investigation began after the German board of education appointed a commission in 1904. Psychologist Oskar Pfungst later tested Hans under controlled conditions, including changing questioners and preventing the horse from seeing them. Hans performed well only when the questioner knew the answer and was visible to him.

Pfungst concluded in 1907 that Hans was reading involuntary shifts in human posture, facial expression, and tension rather than calculating. The finding gave its name to the “Clever Hans effect,” a major warning in animal research: an experimenter's unconscious cues can influence an animal's behavior and make an ability appear more sophisticated than it is. Hans nevertheless showed remarkable sensitivity to human body language.