“What is the sound of one hand clapping?”
Possibly one of the most famous Koan, a Zen riddle given by Zen masters to students to pounder on. Ask this question and see what answer one will give. Observe the use of rationality, eagerness and logic to find the answer of the riddle. One might know from the beginning that the answer does not lie in the use of logic. Actually, if one uses logic, one will always fail to sense the answer (“to understand the answer” is a misguidance of the mind”). If one thinks there is an answer, one fails again.
The key lies in the observation of how hard the mind starts to work to find that answer, and how the ego is eager to prove one’s intelligent and wisdom through deciphering the koan. It is a mind trap!
As far as my beginners mind interpretes the process: if the mind finally gets exhausted (but it doesn’t have to go that far…) , just sit down and be quiet. Be one with silence until you can give up the answer… (this might take years, or less depending on the individual progress) and when you reach the sense of nothingness, detached of all expectations; the sense of the soundless sound of one hand clapping should be felt as if it has always been there. It is like the Zen metaphor of explaining to a fish what the sea is.
I can be totally wrong on this one; As with all koans in Zen , it is about having lived through the process and living it outside the formal meditation space. Whereas I am just thinking out loud.