Boxing is a combat sport in which two people engage in a contest of strength, speed, reflexes, endurance, and will, by throwing punches at each other, usually with gloved hands. Traditionally, the goal has been to knock the opponent unconscious from a blow to the head. Amateur boxing is both an Olympic and Commonwealth sport and is a common fixture in most international games—it also has its own World Championships. Boxing is supervised by a referee over a series of one- to three-minute intervals called rounds. The result is decided when an opponent is deemed incapable to continue by a referee, is disqualified for breaking a rule, resigns by throwing in a towel, or is pronounced the winner or loser based on the judges' scorecards at the end of the contest. While people have fought in hand-to-hand combat since before the dawn of history, the origin of boxing as an organized sport may be its acceptance by the ancient Greeks as an Olympic game in BC 688. Boxing evolved from 16th- and 18th-century prizefights, largely in Great Britain, to the forerunner of modern boxing in the mid-19th century, again initially in Great Britain and later in the United States.