A koan is a multi-level structure device that is used in Buddhism to help a person expand their awareness.
Koans can be long or short, verbal or non-verbal.
At first glance a koan often appears to be a meaningless phrase. However when you focus intently on a koan for extended periods of time, the natural reality can be revealed to you. Koans often juxtapose two or more incongruous concepts. Normally we think that it takes two hand to clap. Your finite mind can think about this, analyze it and get frustrated.