Making his mark

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A man from the state of Chu was a crossing a river. In the boot, his sword fell in to the water. Immediately he made a mark on the boat. "This is where my sword fell off," he said. When the boat stopping moving, he went into the water to look for his sword at the place where he had marked the boat. The boat had moved but the sword had not. Is this not a very foolish way to look for a sword?