empty cup

THE MASTER Nan-in had a visitor who came to inquire about Zen. But
instead of listening, the visitor kept talking about his own
ideas.
After a while, Nan-in served tea. He poured tea into his visitor's
cup until it was full, then he kept on pouring.
Finally the visitor could not restrain himself. "Don't you see
it's full?" he said. "You can't get any more in!"
"Just so," replied Nan-in, stopping at last. "And like this cup,
you are filled with your own ideas. How can you expect me to give
you Zen unless you offer me an empty cup?"

http://www.terebess.hu/english/zen.html