A koan (co-un) is a short story which is a kind of zen buddhist
training to put your mind into a certain state of awareness of
reality. Basically it is a kind of mental or spiritual technology. You
work on it mentally, and your teacher tests you by asking certain
questions until he is satisfied that your mind has attained the
desired state.
Example koans (note you are normally assigned a koan by a Roshi
(teacher), you should not assign them to yourself, but just to give
you an idea):
What is the sound of one hand clapping?
What did your face look like before your ancestors were born?
Does a dog have a buddha nature?
etc.
For popular literature which hits on this kind of stuff read Zen and
the Art Of Motorcycle Maintenance by Robert Pirsig, or for the real
thing, any of the manuals by Roshi Philip Kapleau (sp.?)
-- John.