How Well Do You Know Official NFL Rules?

The offense has four downs with which to gain 10 yards. If they don't, the other team's offense takes over, and they'll attempt to take the ball in the opposite direction.

Each quarter is 15 minutes long. But with stoppages and commercials it often seems more like two hours. In high school football, quarters are only 12 minutes long.

After each play, coaches can substitute a number of players to counter the other team's tactics. Personnel changes can totally change the game's momentum.

Did your cornerback just interfere with a receiver 40 yards downfield? Commence groaning ... because in the NFL, interference penalties are enforced at the spot of the foul.

The play clock is reset to 40 seconds after each play. The offense must snap the ball before that play clock runs out. If they don't, they'll be flagged by the officials.

Offenses are expected to beat the 40-second play clock. When they don't, they are given a five-yard delay of game penalty. Coaches often throw their clipboards in frustration at these penalties.

In the NFL, ball carriers can go to ground and get up again. They're not down until an opponent touches them while they're on the ground.

It's certainly not "safe" for the offense. When defenders tackle the ball carrier in the end zone, they receive a two-point safety ... and to add insult to injury the offense must kick the ball away.

Kickoffs must travel at least 10 yards. And once the ball goes 10 yards, a player from either team can grab it. A very short kick is called an "onside" kick.

It's called the two-minute warning, when there are just two minutes left in a half. It's essentially a free timeout for both sides.

It's not uncommon for teams to remain tied at the end of regulation. In this case, the game goes to overtime and a different set of rules kicks in.

In overtime, if the offense kicks a field goal team also gets a turn on offense. Before 2010, a field goal immediately ended the game. But many fans saw this as an unfair ending to a close game.

Coaches often challenge the outcome of plays by throwing a red flag. But if instant replay upholds the call, the challenging coach loses a timeout.

In overtime, if the offense scores a touchdown, the game immediately ends. But a field goal on the first possession means the other team gets a chance on offense.

A coin flip determined which team got the ball first in overtime. And in most cases, the first team would get just enough yards for a field goal and then end the game ... and the other team's offense never even took the field.

In the NFL, a receiver must land with both feet in bounds. This is different than college football, in which the player needs only one foot in bounds for a legal catch.

After a TD, the offense can go for two points instead of kicking for one. Two-point attempts line up at the two-yard line. And even from this short distance, many two-point attempts fail.

In college, the game clock pauses after a first down. In the NFL, though, the game clock keeps right on going, a fact that dramatically changes coaching strategies.

In a place kick, the ball is snapped to a place holder, and then the kicker boots it ... in hopes of scoring a field goal. Some kickers are accurate from more than 60 yards.

It's hard to protect quarterbacks in the NFL. Offensive linemen are slapped with 10-yard penalties for grabbing and holding charging defenders.

Chop blocks are extremely dangerous and are penalized as such. If the offense commits a chop block in their own end zone, officials declare a safety.

If the kicking team misses, the other team takes possession. The ball is placed at the spot of the kick attempt -- unless it's too close to the end zone. Then, the circumstances change.

All scoring plays are subject to booth review, where officials check instant replay. The idea is to make sure teams aren't unfairly awarded points.

Turnovers can swing the tide of a game. So since 2012, officials use instant replay to confirm each turnover before resuming play. This was one way NFL officials worked to appease outraged fans who clearly saw conflicting events on instant replay.

Sometimes, defenses will block an extra-point attempt and return the ball all the way to the end zone. They don't receive a touchdown ... it's only two points.

After a touchdown, kickers have to earn the extra point. The ball is placed at the 15-yard line. Sometimes, they miss ... a fact that quickly turn the tide of a game.

The referee is the leader of the officiating crew. He spends much of his time observing play in the area of the quarterback.

If both teams kick field goals in overtime, sudden death rules begin. That means the first team to score again wins the game. Just one play can make or break a team.

The game clock doesn't run on PAT kicks. However, the play clock runs as usual, meaning a delay of game penalty can occur.

NFL coaches have two challenge flags at their disposal. But if a challenge is unsucessful they're docked a timeout ... meaning they must carefully weigh challenges.

Intentional grounding (illegally throwing the ball away to avoid a sack) results in 10-yard penalty (or from the spot of the pass, whichever is more). Worse, it costs the offense a down.

It's a rare situation. If NFL coaches successfully use both of their challenges, they are granted still another challenge for use later in the game.

Personal fouls are serious infractions that could result in injury. That's why the offending team is smacked with a 15-yard penalty, a fact that can swing momentum in a hurry.

If an NFL team misses a kick inside the 20-yard line, the other team is granted some breathing room - they get the ball at their own 20.