The following story is attributed to Kermit L. Long:

Two 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.”