Koans arose in China during the Tang Dynasty as Zen masters sought methods to transmit enlightenment experience beyond words and concepts. Unlike riddles with clever answers, koans are designed to frustrate the discursive mind and break through conventional patterns of thought.
Students work intensively with assigned koans during meditation and present their understanding to teachers in formal interview (dokusan). The teacher rejects intellectual explanations, demanding instead an authentic response emerging from direct insight.
The koan serves as a kind of spiritual wrecking ball, demolishing conceptual structures that obscure immediate experience. By wrestling with the impossible question, the practitioner exhausts rational mind and may suddenly glimpse reality beyond subject-object duality.
Classic collections like the Mumonkan ("Gateless Gate") and Blue Cliff Record preserve hundreds of koans that continue challenging seekers to awaken beyond thinking.
Source
Zen Buddhist Tradition