There was a man who sat each day looking out through a narrow vertical opening
where a single board had been removed from a tall wooden fence. Each day a wild
ass of the desert passed outside the fence and across the narrow opening --
first the nose, then the head, the forelegs, the long brown back, the hindlegs,
and lastly the tail. One day, the man leaped to his feet with the light of
discovery in his eyes and he shouted for all who could hear him: "It is
obvious! The nose causes the tail!"