GUTEI'S ONE FINGER
Whenever he was asked the meaning of Zen, Master Gutei held up one finger.
He had a boy attendant who a visitor once asked, "Tell me, what's you master's teaching?"
The boy held up one finger.
Seeing this, Gutei ran over and cut the boy's finger off with a knife.
As the boy ran away screaming, Gutei called out to him. The boy turned his head and Gutei stuck up one finger.
The boy was instantly enlightened.
-- The Gateless Gate, Chapter 3, Mumon. Translation by Koun Yamada Roshi.