Clever Hans was a remarkable horse capable of complex intellectual tasks such as arithmetic, reading, spelling, telling time and even understanding the German language — or at least he had everyone believing that.
The horse would answer questions by tapping his hoof. For instance, if asked a question like: "What's two plus nine?", Hans would stamp his hoof 11 times. Amazingly, questions could be asked verbally or in written form. Hans was almost never wrong.
It was not until a formal investigation by a psychologist in 1907 that Hans was revealed to have been simply reacting to body language cues from his audience. When his audience gasped, anticipating Hans' arrival at the correct answer, he would stop tapping. Still, that's a pretty clever horse!