American football
Football games last for a total of 60 minutes in professional and college play and are divided into two-halves of 30 minutes and four-quarters of 15 minutes. High school football games are 48 minutes in length with two-halves of 24 minutes and four-quarters of 12 minutes. The two-halves are separated by a halftime period, and the first and third quarters are also followed by a short break. Prior to the start of the game, the referee and team captains for each team meet at midfield for a coin toss. The visiting team is allowed to call 'heads' or 'tails'; the winner of the toss is allowed to decide between choosing whether to receive or kick off the ball or choosing which goal they want to defend, but they can also defer their choice until the second half. The losing team, unless the winning team decides to defer, is allowed to choose the option the winning team did not select, and receives the option to receive, kick, or select a goal to defend to begin the second half. Most teams choose to receive or defer, because choosing to kick the ball to start the game would allow the other team to choose which goal to defend. Teams switch goals following the first and third quarters. If a down is in progress when a quarter ends, play continues until the down is completed.
Running out the clock
In both professional and college football, the offense has 40 seconds (in the Alliance of American Football, 30) from the end of the previous play to run the next play. A team running out the clock will allow the play clock (which records the time remaining until a play must be run) to drain as much as possible before running its next play. In the NFL, this is particularly noteworthy due to the existence of the two-minute warning. If the trailing team has no timeouts remaining and the leading team is in possession of the ball with a first down at the two-minute warning, they can effectively run out the clock and win the game without running another positive play. With two minutes to go (120 seconds), the offense can take three "knees", one each on 1st, 2nd, and 3rd down (using all 40 seconds from the play clock on each), and allow the game clock to expire before having to run a play for fourth down. A similar situation can be had by also achieving a first down inside the two-minute warning. This practice is commonly known as the "Victory Formation", as the offense lines up in a tightly-protective "V" formation to minimize the chances of a fumble or other turnover. (The AAF lets teams run out the clock on three straight victory formations from 90 seconds left in regulation.)