Zen koans are a form of riddle used to test and increase a Zen student's enlightenment. These are stories or questions that may not have an obvious "answer." They are designed to force the listener to think about the universe in a different way.
One common Zen koan is, "What is the sound of one hand clapping?" It is meant to force the student to consider duality. Ultimately the seeker and the answer become one. Many koans have no true answer. Other answers may seem paradoxical. The purpose is to exhaust the student's intellect and make him more receptive of understanding and enlightenment.