It is said that there are nearly 1,700 kung-an (Japanese: koan) that Zen masters used.  In Zen, practitioners use kung-an as subjects for meditation until their minds come to awakening.  The difference between a kung-an and a math problem lies in the fact that while the solution of the math problem lies in the problem itself, the response to the kung-an lies in the life of the practitioner.  The kung-an is not an enigma to resolve, and we cannot define it as a theme or subject of meditation.  A kung-an is only a skillful means to help a practitioner reach his goal.  It can also be an obstacle to awakening for a practitioner who thinks that truth is hidden in the kung-an and can be interpreted in conceptual terms.

Kung-an were popular during the T’ang Dynasty in China.  Before this time, Zen masters did not use kung-an.  Therefore we can conclude that kung-an are not indispensable to the practice of Zen. 

Ten Thousand Sam?dhi-s (poem by Zen Master Hahn Am)

Deep pine tree valley:

Sitting quietly,
The moon was bright last night.
The ten thousand sam?dhi-s are not necessary.
When thirsty, drink.
When tired, sleep.

Quiet Night, the Geese Cry

Sitting silently in a mountain temple in the quiet night,
Extreme quiet and stillness are original naturalness.

Why does the Western wind shake the forest?
A single cry of the cold-weather geese fills the sky.