The Answer:

The nautical knot finds its origin in a clever method sailors once used to measure their speed at sea. They would tie knots in a long line at regular intervals, then cast one end (weighted down by a floating piece of wood) out behind their vessel. The number of knots let out in a given period of time (measured, usually, by a small hourglass) would allow them to calculate the distance and consequently the speed at which the boat was moving. One knot is one nautical mile (about 1.15 statute miles) per hour.

—The Editors