Zen Koan "The monk and the staff"

A monk approached a Zen Master carrying a staff.
"Drop it!" the roshi shouted. The monk let the staff fall to the floor.
"Drop It!" the roshi shouted again.
"I have nothing to drop," the bewildered monk replied.
"Drop it!" the roshi said forcefully.
The monk was enlightened.

-Zen Flesh Zen Bones