an american football game is played with a ball called a football. the sport is played on a grass (or artificial turf) field. the field is exactly long and wide. the end zones, one at each end of the field, are long. the players wear protective equipment, including a helmet with face mask, shoulder pads, thigh pads, and often a mouth guard. a team has four chances, or "downs", to advance the ball while the opposing team tries to stop it. certain players advance the ball by carrying or throwing it. teams can score by advancing the ball to the other team's end zone or by kicking the ball through a goalpost placed at the back of the end zone. american football is carefully regulated by time and rules, which are enforced by officials, who also determine when a team scores.