A man from the state of Chu was crossing a river. In the boat, his sword fell into the water due to his carelessness.
Immediately, he made a mark on the side of the boat. "This is where my sword fell in," he said.
When the boat finally reached the shore and stopped, he jumped into the water to search for his sword at the exact spot he had marked.
The boat had moved a great distance, but the sword had not. Is this not a very foolish way to look for a lost sword?