Kōan

The kōan (; ) are groupings of related questions and answers which are a paradox. The "kōan" may be a story which cannot be understood or explained easily.
The "kōan" can be a special kind of metaphor with a hidden meaning, such as "the sound of one hand clapping".
History.
"Kōans" originate in the sayings and events in the lives of wise men and legendary figures. 
A "kōan" can refer to a story selected from Buddhist historical records and sutras.
The "kōan" is a fundamental part of the history and practice of Zen Buddhism.