In a city of unending traffic there lived a crow with a passion for walnuts and no means of opening them. The shells defied its beak, and for weeks it prowled the parking lots, hungry and increasingly indignant, like a gourmet locked out of his own restaurant. Then, one afternoon, it watched a car roll over a fallen walnut, heard a sharp crack and saw the shell split open like a verdict. The crow, who had never attended a physics lesson, grasped the principle at once.
From then on it worked with the patience of a craftsman. It carried a walnut to the zebra crossing, laid it in the path of the traffic and withdrew to the traffic light to wait. The wheels did the heavy lifting. Only when the light turned red, the engines fell quiet and the pedestrians began to stream across did the crow descend, with the studied nonchalance of a man who merely happens to be passing, and collect its lunch from between the commuters' feet.
An old man named Mr. Sato, who fed sparrows on the bench beside the crossing, observed this routine every day and at first declined to believe it. Within weeks he had become an accomplice, carrying walnuts from home and setting them carefully on the tarmac like a man laying a table. Years later the crow came back to the crossing with two fledglings, and the lesson began again, with the whole city as its unwitting classroom.