Once, a professor went to a Zen Master. He asked him to explain the meaning of Zen. The Master quietly poured a cup of tea. The cup was full but he continued to pour.
The professor could not stand this any longer, so he questioned the Master impatiently, "Why do you keep pouring when the cup is full?" "I want to point out to you," the Master said, "that you are similarly attempting to understand Zen while your mind is full. First, empty your mind of preconceptions before you attempt to understand Zen."