We've already learned about the two ways that a team can move the pigskin in football.
Any player that has possession of the ball is called the ball-carrier.
Defensive players can stop the forward-progress of the ball-carrier by tackling him to the ground.
A tackle is the act of bringing a player to the ground by force.
A sack is when a defensive player tackles the quarterback behind the line of scrimmage for a loss of yards.
The Player Is Down
In NFL football, the ball-carrier is considered tackled or downed when both of his knees touch the ground.
In NCAA college football, the ball-carrier is considered tackled or downed when at least one knee touches the ground.
In either case, when the ball-carrier is tackled to the ground, the scrimmage down ends.