The muddy road koan:

Tanzen and Ekido were once travelling together down a muddy road. A heavy rain was still falling.
Coming around the bend, they met a lovely girl in a silk kimono and sash, unable to cross the intersection.
“Come on girl”, said Tanzen at once. Lifting her in his arms, he carried her over the mud.
Ekido did not speak again until that night when they reached a lodging temple. Then he no longer could restrain himself. “We monks don’t go near females,” he told Tanzen, “especially not young and lovely ones. It is dangerous. Why did you do that?”
“I left the girl there,” said Tanzen. “Are you still carrying her?”

Koan: /koh’an/ n. A Zen teaching riddle. Classically, koans are attractive paradoxes to be meditated on; their purpose is to help one to enlightenment by temporarily jamming normal cognitive processing so that something more interesting can happen (this practice is associated with Rinzei Zen Buddhism).

I originally ran across this Zen koan in An Index of Possibilities: Energy & Power by John Chesterman ISBN: 0-394-73037-2.