Koan are stories, often in the form of questions and answers, set as problems for meditation in the practice of Zen Buddhism, although the problems are not for solving by linear or rational processes. The most famous is the problem of the noise made by one hand clapping. The mu-koan is the reply mu (meaning nothing) given by the master Joshu in answer to the question whether a dog has the nature of Buddha. Another is the interchange: ‘What is Buddha?’ ‘Three pounds of flax.’