Two men were walk in downtown New York on a crowded street. One said to the other, “Listen, it’s the sound of a cricket.”
The other heard nothing. He asked his companion how he could detect the cricket’s sound through all the noise.
The first man, a zoologist, was trained to hear the marvelous sounds of nature. Reaching into his pocket, he pulled out a coin and immediately dropped it. A dozen people heard the sound and began to look all around.
“We hear,” he said, “what we listen for.”