Try searching for the sound of one hand clapping!

"...in the beginning a monk first thinks a k?an is an inert object upon which to focus attention; after a long period of consecutive repetition, one realizes that the k?an is also a dynamic activity, the very activity of seeking an answer to the k?an. The k?an is both the object being sought and the relentless seeking itself. In a k?an, the self sees the self not directly but under the guise of the k?an... When one realizes ("makes real") this identity, then two hands have become one. The practitioner becomes the k?an that he or she is trying to understand. That is the sound of one hand." — G. Victor Sogen Hori, Translating the Zen Phrase Book.