It is generally accepted that Zen started in China (originally called "Chan" before it became "Zen" in Japan) with the monk Bodhidharma so his audience with the Chinese Emperor seems like a good place to start.
He was asked three questions by the Chinese Emperor who considered himself to be very knowledgeable in Buddhism. Here is an account of the whole story from a Zen site:
https://www.thezengateway.com/teachi...hinese-emperor
Although all of Bodhidharma's responses have depth (to the consternation of the Chinese Emperor), the response to the third question has been of particular interest to me personally.
Bodhidharma was reportedly asked: "Who is it then that stands before us?” In other words, "Who Are You"?
Bodhidharma's response was in itself a very deep koan. His response was: "Not Known" with alternative translations being "Not Knowing" (my personal preference) and "I do not know".
How would you interpret Bodhidarma's response? It apparently was not well received by the Chinese Emperor for obvious reasons.