In a city of endless traffic there lived a crow with a passion for walnuts and no means of opening them. The shells were far too hard for its beak, and for weeks it hopped between parking lots, hungry and increasingly indignant. Then, one afternoon, it watched a car roll over a fallen walnut, heard a sharp crack and saw the shell split open. The crow, who had never taken a physics lesson, grasped the principle at once.
From then on it worked with the patience of a professional. It carried a walnut to the zebra crossing, dropped it in the path of the traffic and withdrew to the traffic light to wait. The wheels did the hard part. Only when the light turned red, the cars stood still and the pedestrians began to stream across did the crow hop down, with the studied indifference of a man who happens to be passing, and collect its lunch.
An old man named Mr. Sato, who fed sparrows on the bench beside the crossing, observed this routine every day and at first refused to believe it. After a few weeks he had become a collaborator, bringing walnuts from home and placing them carefully on the road. Years later the crow came back to the crossing with a pair of fledglings, and the lesson began again.