How well do you know the rules of football?

Players often (accidentally or intentionally) grab the facemask of an opponent's helmet during plays. Because its so perilous to a player's health, an intentional facemask infraction is penalized with a 15-yard penalty.

The front of the end zone is marked by the goal line. The offensive team expends every bit of energy in an attempt to cross the goal line to score a touchdown. In the case of the Cleveland Browns, this doesn't often occur.

Each side is allowed a maximum number of 11 players on the field at a time. Exceed that number and you'll be penalized for having too many players on the gridiron.

Each quarter of the game is 15 minutes long. That's football time, of course, so clock stoppages and commercial breaks add to the total.

In the NFL, coaches can challenge several official calls during a game. Coaches throw their red flags when then want to challenge a call. If they lose the challenge, their team forfeits a timeout.

The worst (and typically most dangerous) penalties are punished with the longest yardage. The longest yardage penalties are 15 yards. Examples are infractions like blocking below the waist or facemask.

When the center "snaps" the football to the quarterback (or kicker), it's the start of the offensive play. A "bad snap" means that the exchange is bungled, which sometimes results in a turnover (and always ends in a coach's ire).

A touchdown is worth six points. The scoring team then has the option of kicking an extra point or attempting to go for a two-point conversion.

The offense gets four downs to gain 10 yards. In most situations, if the offense fails to achieve a first down after three plays, it will punt the ball away, meaning the other offense will get the ball and a new chance to drive the field and score.

The line of scrimmage is the imaginary line that divides the offense and defense at the beginning of each play. Coaches often applaud (or bemoan) their team's ability (or inability) to control the line of scrimmage.

Illegal motion is the penalty that officials call if an offensive player is moving towards the line of scrimmage before the play begins. It's a five-yard penalty.

The kicking unit is often called "special teams." They are specifically designed to maximize the team's abilities on kickoffs, punts and field goal attempts. Special teams players typically aren't on the field as often as offensive and defensive players, but their skills are equally important.

The arms-straight-up signal is the ultimate gesture of triumph in football. It means that a team has either successfully scored with a kick, or better yet, driven the field for a touchdown.

Most modern football teams use the quarterback for passing plays. But once in a while (particularly during trick plays), other players will get a chance to heave the football downfield.

For pro football kickoffs, the ball is placed at the defense's 30-yard line. Powerful kickers often blast the kickoff all the way through the opponent's end zone, making it impossible for the offense to return the kick.

American football fields are 100 yards long, not including the end zones. The field is a little more than 53 yards wide.

Tight ends play offense. They are usually a lot like big wide receivers -- bulky enough to create large blocking lanes but agile enough to run precise receiver routes.

A team that grabs the ball and breaks the plane of the goal line scores a touchdown, that most coveted of football scores. Both college and NFL football often use instant replay cameras to determine whether a player has broken the plane of the goal line.

Spearing (or spear tackling) refers tackles in which players lead with their helmets, driving their bodies into an opposing player in a spear-like fashion. It's incredibly dangerous and very much against the rules.

If an offensive player is tackled behind his own goal line, it results in a safety for the opposing team. The defense gets two points ... and then possession of the ball via a kick by the offensive team.

Each half of the game begins with a kickoff, so there's always at least two. Kickoffs also follow each score.

Before the offense begins a play, at least seven players must be right on the line of scrimmage. The other four players must be lined up at least one yard away from the line of scrimmage.

The diamond-shaped gesture is a sure sign that the defensive team has scored a safety. Safeties are rare, but they're worth two points, and it means that the scoring team will immediately get the ball again, too.

The clock doesn't stop if the quarterback is sacked -- unless, of course, the sack pushes the quarterback out of bounds or into the end zone.

If a defensive player crosses the line of scrimmage before the snap, he's flagged for encroachment. This is a five-yard penalty and it nullifies the play.

Each football team is made up of three basic units -- the offense, defense and kicking units. Teams that neglect one or more of those units nearly always pay for it with a loss during the season.

It's true that modern football has specific defensive specialty positions, such as cornerback, safety and defensive end. But defensive players are not required to line up at particular spots on the field. Offensive players, however, much follow stricter guidelines.

Anytime a player grabs an opponent (other than the ball carrier) they may be called for a holding penalty. Due to the physical nature of the game, holding is one of the most common penalties ... and one of the most frustrating for fans to witness.

There are five different ways that teams can score during a game. They are touchdown, field goal, two-point conversion, extra point, or safety. Touchdowns, of course, are the most desirable because they're worth six points, which is double the points of a measly field goal.