The clock stops after certain plays; therefore, a game can last considerably longer (often more than three hours in real time).
If an NFL game is tied after four quarters, the teams play an additional period lasting up to 15 minutes. In an NFL overtime game, the first team that scores, wins, ending the game.
In a regular-season game, if neither team scores, the game is a tie. In a post-season game, play continues with additional overtime periods until the tie is broken, as in the 1971 Christmas Day double-overtime game between the Miami Dolphins and Kansas City Chiefs, the longest game in NFL history.
College overtime rules are more complicated and are described at Overtime (sport).