<P> Games last longer than their defined length due to play stoppages--the average NFL game lasts slightly over three hours . Time in a football game is measured by the game clock . An operator is responsible for starting, stopping and operating the game clock based on the direction of the appropriate official . A separate clock, the play clock, is used to determine if a delay of game infraction has been committed . If the play clock expires before the ball has been snapped or free - kicked, a delay of game foul is called on the offense . The play clock is set to 40 seconds in professional and college football and to 25 seconds in high school play or following certain administrative stoppages in the former levels of play . </P> <P> There are two main ways that the offense can advance the ball: running and passing . In a typical play, the quarterback calls the play, and the center passes the ball backwards and under his legs to the quarterback in a process known as the snap . The quarterback then either hands the ball off to a back, throws the ball or runs with it himself . The play ends when the player with the ball is tackled or goes out of bounds, or a pass hits the ground without a player having caught it . A forward pass can only be legally attempted if the passer is behind the line of scrimmage . In the NFL, a down also ends if the runner's helmet comes off . </P> <P> The offense is given a series of four plays, known as downs . If the offense advances ten or more yards in the four downs, they are awarded a new set of four downs . If they fail to advance ten yards, possession of the football is turned over to the defense . In most situations, if the offense reaches their fourth down they will punt the ball to the other team, which forces them to begin their drive from further down the field; if they are in field goal range, they might also attempt to score a field goal . A group of officials, the chain crew, keeps track of both the downs and the distance measurements . On television, a yellow line is electronically superimposed on the field to show the first down line to the viewing audience . </P> <P> There are two categories of kicks in football: scrimmage kicks, which can be executed by the offensive team on any down from behind or on the line of scrimmage, and free kicks . The free kicks are the kickoff, which starts the first and third quarters and overtime and follows a try attempt or a successful field goal, and the safety kick, which follows a safety . </P>

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