In 1904 a horse in Berlin called Hans was answering arithmetic by tapping a hoof, and a panel of experts looked hard and found no trickery. In 1907 the psychologist Oskar Pfungst found the trick nobody was playing. Hans got the sum right when the person asking knew the answer and stood where he could see them, and almost never otherwise. As the taps neared the right number the questioner tightened very slightly, and at the right one they relaxed, and Hans stopped. None of them knew they were doing it. Pfungst learned to read the cue himself, then found he couldn’t stop giving it, even knowing exactly what it was. Hans never learned to count. He learned something harder: the moment a person stops holding their breath.