What a koan is
The word comes from the Chinese gong'an, "public case," a legal term: a precedent to be examined. A koan is usually a short exchange between a Zen master and a student, preserved because something in it defeats ordinary logic. The most famous: a monk asks master Zhaozhou (Joshu) whether a dog has Buddha-nature. Doctrine says yes, all beings do. Zhaozhou answers "Mu," roughly "no," or "un-ask the question." That single syllable has been assigned to Rinzai students as their first koan for eight centuries.
What koans are for
Not for solving. The student sits with the koan through months of zazen, brings answers to the teacher in private interviews, and has them rejected, sometimes wordlessly, until the clever machinery runs out. What the koan attacks is the assumption that understanding always arrives as a proposition. The breakthrough it aims at is a shift in the one doing the understanding.
An analogy that mostly survives: you can't learn balance from a lecture about bicycles. The koan is the wobbling bicycle. The teacher checks whether you're riding it or describing it.
The famous cases
"Two hands clap and there is a sound; what is the sound of one hand?" comes from Hakuin, the 18th-century reviver of Rinzai Zen, who made it a standard first assignment alongside Mu. The classic collections are The Gateless Gate (Mumonkan, compiled by Wumen in 1228, 48 cases) and the larger, more literary Blue Cliff Record. Other standards: "What was your original face before your parents were born?" and Zhaozhou again, answering a question about ultimate meaning with "The cypress tree in the garden."
Do you need koans to practice Zen?
No. Koan training is the specialty of the Rinzai school and is done under a teacher, since a koan you grade yourself is just a puzzle you've decided you solved. Soto Zen, the other main stream, mostly sets koans aside in favor of shikantaza, bare sitting; the split is covered in our What is Zen guide. A beginner's honest options are sitting practice (our five-minute method) and reading the cases as literature, which they reward.
Reading them
Start with the beginner Zen books; for the cases themselves, any annotated Gateless Gate serves. And 365 Days of Zen threads koans and the masters behind them through a year of one-page days, which is roughly the pace koans prefer: one strange thing, carried around until it opens.
Common questions
Do koans have answers?
Not answers you can look up. In formal training a teacher accepts or rejects a student's response based on what it shows, not what it states. Published 'answer books' exist and are considered beside the point.
What does Mu mean?
Literally 'no' or 'not,' but Zhaozhou's Mu functions as a refusal of the question's yes/no frame. Students are told to carry the syllable itself, not analyze it.
Can I work on koans without a teacher?
You can read and sit with them, and they're worth reading. Formal koan training, though, is defined by the teacher's checking; without it the tradition says you're grading your own exam.
What's the best collection of koans to read?
The Gateless Gate (Mumonkan, 1228): 48 cases with Wumen's commentary, short enough to live on a nightstand. The Blue Cliff Record is the bigger, more ornate sequel.