From Nonverbal Communication in Human Interaction by Mark L. Knapp
Clever Hans was a horse whose owner became convinced that he was capable of doing math. The owner would ask the horse: what is 4/2 and the horse would tap twice. As the questions became more and more difficult and the horse was apparently capable of doing factorials in his head, he garnered the attention of the world community. Many people flocked to watch the horse perform complex arithmetic in his horsey mind and even many in the scientific community became convinced that the horse in fact was a master-class arithmetist, and that they had been underestimating animal intelligence by a vast magnitude.
It turned out however that if the math question was spoken into the horse's ear and their was no one in his visual range that the horse could not come up with correct answer. He was reading the body-language of the audience which was unconsciously signaling to Hans when to stop tapping!