Having the NFL back is a joy. Having to deal with NFL referees again is not.

Refereeing is a hard job but sometimes they get things obviously wrong. Fortunately, there is relief for NFL coaches who can throw the red flag to indicate they want to challenge a ruling.

How does challenging work in the NFL?

NFL challenge rules: How many coaches challenges per game?

If a coach believes the refs missed a call, he can challenge the play, but there are limitations on that power.

Every team gets at least two challenges per game. If they are successful on the first two challenges, they can have a third.

However, the team must have a timeout available in order to challenge. If the challenge is unsuccessful, they are charged that timeout. If it is successful, the timeout goes back in their pocket.

NFL challenge rules: How does a booth review work?

In the final two minutes of either half, the replay official or NFL replay command center in New York can instigate a replay review if they spot a play that needs to be reevaluated.

In addition to that, every scoring play, turnover and player disqualification is automatically reviewed by the replay official.

NFL challenge rules: What can and can't be reviewed by replay?

Replay can be used to determine a wide array of things on the football field, from possession to field position and out-of-bounds determinations to disqualifications. It's easier to look at what can't be reviewed.

From the official NFL rule book, these elements are not eligible for review:

(a) Whether an erroneous whistle sounded;
(b) Whether a ball was illegally batted or kicked;
(c) Whether a passer intentionally grounded a pass;
(d) Whether an ineligible receiver was downfield before a pass;
(e) Whether a receiver was illegally contacted;
(f) The spot of a loose ball crossing the sideline;
(g) Whether a block was illegal; and
(h) Any aspect of a play not listed as reviewable in Section 3 of this Rule.

So there you have it. Those are the things a coach can't challenge and replay can't review.