A monk asked Joshu in all earnestness.
“What is the meaning of the patriarch's coming from the west?”
Joshu said,
"The oak tree there in the garden.”
---
There is a wonderful story about this koan.
Shido Munan Zenji was travelling along the Tokaido road from Kyoto to Edo.
He was being followed by a robber who put up at the same inn as Shido, planning to rob him during the night.
The thief opened the door and peeked in.
To his astonishment, what he saw there in the room was a garden with an oak tree in it.
Suddenly, he heard a voice exclaim, “Who's there?”
The tree transformed into Shido sitting in meditation.
The thief was so stunned that he apologized and asked to learn about this amazing technique that allowed him to change into an oak tree.
The teacher taught him to do zazen and gave him a koan.
In time, the man changed and lived an honest life.
[From the Mumonkan, Case 37]