Zen koans are very difficult to write about because the point of koan practice is to cut through the mental concepts and linear thinking that usually dominate our mind. So how do you describe what koans are, without falling into this trap? That is a koan in itself.
Koans are usually described as puzzles or seemingly nonsensical statements that highlight the ways we normally think and perceive the world. They are not meant to be answered but rather to be contemplated, and doing so can create a shift in perception that allows us a glimpse of pure awareness or 'buddha mind'. Koans often seem paradoxical, but with contemplation can show us the assumptions in our thinking and relationship with the world that actually create our sense of paradox.
The idea behind koan practice is to witness our own mind or transcend dual thinking. A breakdown of our usual distinction between subject and object, perceiver and perceived, is often said to be at the heart of one of the most well-known Zen koans, "what is the sound of one hand clapping?", or its more literal translation:
Two hands clap and there is a sound. What is the sound of one hand?
— Hakuin Ekaku
Most Zen schools of Buddhism employ koan practice to some extent, but it is most closely associated with Rinzai Zen. Aspiring teachers in Rinzai lineages are tested on their ability to use the Rinzai koan collections when working with students. There are many compilations of Zen koans, the most famous of which are the Blue Cliff Record and Book of Equanimity (both compiled in the 12th century), and The Gateless Gate and Collection of Stone and Sand (both compiled in the 13th century).
Teaching through koans can take many forms. Sometimes Zen teachers will give a talk about a koan, or create their own parallel koans or poems. Sometimes a teacher will give a student a koan to contemplate, or use a koan as a litmus test for realization. A student might work with one koan for months or even years, returning to the teacher many times to comment on their koan. Again, koans do not have 'answers' in the traditional sense, so a teacher is not looking for a particular response. The purpose is for true realization to occur, and the expression of that realization can take many forms. In traditional parables that relay such teacher-student exchanges, the student often responds with sounds, movements, or other unexpected actions, as in this story:
Mokugen was never known to smile until his last day on earth. When his time came to pass away he said to his faithful ones: "You have studied under me for more than ten years. Show me your real interpretation of Zen. Whoever expresses this most clearly shall be my successor and receive my robe and bowl."
Everyone watched Mokugen's severe face, but no one answered.
Encho, a disciple who had been with his teacher for a long time, moved near the bedside. He pushed forward the medicine cup a few inches. That was his answer to the command.
The teacher's face became even more severe. "Is that all you understand?" he asked.
Encho reached out and moved the cup back again.
A beautiful smile broke over the features of Mokugen. "You rascal," he told Encho. "You worked with me ten years and have not yet seen my whole body. Take the robe and bowl. They belong to you."
- Taken from Collection of Stone and Sand (a different translation of this story is available in Zen Flesh Zen Bones - see below)
Some Zen Haiku poems are considered koans, and other koans are in a story or parable form, where a teacher or master teaches in the moment, as in this case:
Two monks were arguing about a flag. One said: 'The flag is moving.'
The other said: 'The wind is moving.'
The sixth patriach happened to be passing by. He told them: 'Not the wind, not the flag; mind is moving.'
- From The Gateless Gate collection
Another parable-style koan is The Moon Cannot Be Stolen, from the Collection of Stone and Sand:
Ryokan, a Zen master, lived the simplest kind of life in a little hut at the foot of a mountain. One evening a thief visited the hut only to discover there was nothing to steal.
Ryokan returned and caught him. "You have come a long way to visit me," he told the prowler, "and you should not return empty-handed. Please take my clothes as a gift."
The thief was bewildered. He took the clothes and slunk away.
Ryoken sat naked, watching the moon. "Poor fellow," he mused, "I wish I could have given him this beautiful moon."
For a well-rounded collection of different types of koans, which includes koans from both The Gateless Gate and Collection of Stone and Sand collections, try:
Or, if you prefer e-books, note that this article is included in my e-book Introduction to Buddhism and Buddhist Meditation.