men were walking along a crowded sidewalk in a downtown business area.
Suddenly one exclaimed: "listen to the lovely sound of that
cricket." But the other could not hear. He asked his companion
how he could detect the sound of a cricket amid the din of people and traffic.
The first man, who was a zoologist, had trained himself to listen to the voices
of nature. But he didn't explain. He simply took a coin out of his
pocket and dropped it to the sidewalk, whereupon a dozen people began to look
about them. We hear," he said, "what we listen for."