Football is a team sport that is played all over the world. Millions of people regularly attend professional games and billions watch the game on television.
The game is played on a field called the gridiron, which has stripes running down the width of it at 5-yard intervals and hash marks marking each single yard. Teams play four 15-minute quarters, separated by a 12-minute break at halftime.
Players on each team are divided into three positions: quarterback (QB), offensive linemen and linebackers. The quarterback calls the plays and yells signals at the line of scrimmage. He then hands off the ball to a running back, throws it to a receiver or runs with it himself. The offensive line provides blocking for the quarterback and running backs by consisting of a center, guards and tackles.
A player runs out of bounds when his body touches any part of the ground other than the line or endline (e.g., if his foot hits the ground while he is running across the sideline). When this happens, a play is over and the ball and player are awarded the first down.
Penalties are issued by referees for rule violations, including fumbling, holding, and hitting the ball too hard. If a penalty causes the ball to go out of bounds, the team that has possession gets a free kick from where the play ended.
In addition to the above, a team can win the game by scoring a touchdown or if the opponent scores a safety. A safety is a rare occurrence where the ballcarrier is tackled, forced out of bounds, loses the ball out of bounds or commits certain penalties in his own end zone.