The Gateless Gate/Joshu's Dog
A monk asked Joshu, a Chinese Zen master: "Has a dog Buddha-nature or not?"

Joshu answered: "Mu." 
 Mumon's comment: To realize Zen one has to pass through the barrier of the patriarchs. Enlightenment always comes after the road of thinking is blocked. If you do not pass the barrier of the patriarchs or if your thinking road is not blocked, whatever you think, whatever you do, is like a tangling ghost. You may ask: What is a barrier of a patriarch? This one word, Mu, is it.
 This is the barrier of Zen. If you pass through it you will see Joshu face to face. Then you can work hand in hand with the whole line of patriarchs. Is this not a pleasant thing to do?
 If you want to pass this barrier, you must work through every bone in your body, through every pore of your skin, filled with this question: What is Mu? and carry it day and night. Do not believe it is the common negative symbol meaning nothing. It is not nothingness, the opposite of existence. If you really want to pass this barrier, you should feel like drinking a hot iron ball that you can neither swallow nor spit out.
 Then your previous lesser knowledge disappears. As a fruit ripening in season, your subjectivity and objectivity naturally become one. It is like a dumb man who has had a dream. He knows about it but he cannot tell it.
 When he enters this condition his ego-shell is crushed and he can shake the heaven and move the earth. He is like a great warrior with a sharp sword. If a Buddha stands in his way, he will cut him down; if a patriarch offers him any obstacle, he will kill him; and he will be free in his way of birth and death. He can enter any world as if it were his own playground. I will tell you how to do this with this koan:
 Just concentrate your whole energy into this Mu, and do not allow any discontinuation. When you enter this Mu and there is no discontinuation, your attainment will be as a candle burning and illuminating the whole universe.
 Has a dog Buddha-nature?
 This is the most serious question of all.
 If you say yes or no,
 You lose your own Buddha-nature.

See also: article on Mu at Wikipedia.