Here's why:

* Knots are nautical miles per hour. A nautical mile is approximately 1.15 statute miles (land miles).

* Historically, knots were measured by throwing a log (a piece of wood) overboard with a rope attached. The rope had knots tied at regular intervals. The speed was determined by counting how many knots passed through the hand in a specific amount of time.

* Today, electronic instruments are used to measure speed, but the unit "knot" is still used for consistency and tradition.

So, when you hear someone say a ship is traveling at 20 knots, they mean it's going 20 nautical miles per hour.