A Real Life Zen Koan...

From the Wikipedia article on John Cage's piece 4'33":

In 1951, Cage visited the anechoic chamber at Harvard University. An anechoic chamber is a room designed in such a way that the walls, ceiling and floor absorb all sounds made in the room, rather than reflecting them as echoes. Such a chamber is also externally sound-proofed. Cage entered the chamber expecting to hear silence, but he wrote later, "I heard two sounds, one high and one low. When I described them to the engineer in charge, he informed me that the high one was my nervous system in operation, the low one my blood in circulation."

4'33" is John Cage's most famous work, where the musician or musician plays nothing for 4 minutes and 33 seconds. It's often described as four minutes and thirty-three seconds of silence.

But that's completely wrong -- it's a piece about the impossibility of silence. Even in the most silent place on Earth, you still hear the rhythms of your blood.