What are we listening for?

Two friends were walking down a crowded city street. All around them, buses roared, horns beeped, and trams rattled.

The first man, a zoologist, turned to his friend, and said " Can you hear those cicadas?"

His friend exclaimed "What?! How can you hear cicadas amongst all this noise?"

The first man took some coins out of his pocket and dropped them on the pavement. Nearly every head in the crowd turned to look where the sound had come from.

He said to his friend: "See? We only hear what we listen for..."