One Way to Count Cows
There is, you may recall, an old tale about a ranch owner of Montana who was returning home by train. En route he got into a conversation with a fellow travelor, who astonished him by stating categorically the number of cattle in each of the numerous herds they were passing. It happened that the train was nearing the cattleman's station and that they were coming to a pasturage of his own, containing a herd of cows which had been counted just a few weeks before. To his amazement, his fellow traveler, after a quick glance and a moment of thought, gave him the exact number. With a few words of congratulation, the Cattleman asked him how he did it. "Oh," said the Lightning Calculator, "I just count the number of legs and divide by four."
Source: Denning Miller's Popular Mathematics, 1942, p. 180