All this happened, more or less.

Once Nan-in, a Japanese master of the Meiji era, received a university professor who came to inquire about Zen.

Nan-in served tea to the guest. He poured his cup full, and then kept on pouring.

The professor watched the overflow until he no longer could restrain himself.
"It is overfull! No more will go in!"

"Like this cup," Nan-in said, "you are full of your own opinions and speculations.
How can I show you Zen unless you first empty your cup?"

via centralasian

http://www.amazon.com/Zen-Flesh-Bones-Collection-Writings/dp/0804831866