Running out the clock
In American football, each quarter of a game is measured with a 15-minute game clock, or 12-minute clock in many high school football codes and the German Football League. A team in possession of the lead and the ball will attempt to use up as much of the game clock as possible in order to bring the game to an end more quickly, thus denying the opposition another chance on offense.
American football rules
Teams change ends of the field at the end of the first quarter and the end of the third quarter, though otherwise the situation on the field regarding possession, downs remaining and distance-to-goal does not change at these occasions (so a team with possession 5 yards from the opponent's endzone at the end of the first quarter would resume play 5 yards from the endzone at the other end of the field, which they would then be attacking) . Separating the first and second halves is "halftime." Both halves, and any overtime, begin with kick-offs — the kicking team is decided by a coin toss (see below).