The Lost Sword

A man was crossing the river when he accidentally dropped his sword into the river. Immediately he made a mark on the boat where the sword had fallen. "This is where my sword fell off, I shall find my sword by this mark." he said.

When the boat stopped moving, he went into the water to look for his sword at the place where he had marked the boat. Needless to say, it was useless. The boat had moved but the sword had not. He was unable to find the lost sword.