The most famous of the Elberfeld horses, trained by the retired German mathematics teacher Wilhelm von Osten (1838–1909) in 1901 and displayed in Berlin for several years, appearing to have a good knowledge of spoken and written German and an ability to tell the time and carry out mental arithmetic, among other equally unlikely capabilities. Clever Hans (der kluge Hans) answered yes/no questions by tapping its hoof on the ground according to a prearranged code, and fraud seemed unlikely, because von Osten allowed anyone to watch the horse perform and even to put questions to the horse in his absence, but the psychologist Oskar Pfungst (1874–1932) studied the horse experimentally and discovered that its abilities were illusory. Clever Hans could not respond intelligently when no one within its visual field knew the right answer to the question being asked: for example, if one person whispered a number into its left ear and another person a different number into its right ear, then if Clever Hans was asked to add the two numbers, it was stumped. Pfungst concluded that Clever Hans had learnt to respond to non-verbal communication: it began tapping whenever people standing nearby adopted expectant postures and turned their attention to its hoof, and it stopped as soon as the onlookers (probably unwittingly) gave anticipatory head and eye movements when the correct number of taps was reached. Von Osten usually wore a wide-brimmed hat, which had the effect of amplifying small head movements and making the horse's task easier, but he was probably unaware of this. In 1907 Pfungst published an account, the English translation of which was entitled Clever Hans (the Horse of Mr von Osten): A Contribution to Experimental and Animal Psychology (1911). See also facilitated communication.