- A timeout is called (duh, right?)
- The person with the ball runs out of bounds (this is important when there's not much time left in the half)
- There's an injured player on the field (although sometimes the team has to use a timeout then too - it depends)
- The red challenge flag is thrown by a coach (which means the referees review the previous play to see if the decision needs to be changed)
- It is the 2-minute warning (this is at the end of the 2nd and 4th quarters)
- Change of possession (interception, score, punt, etc.)
Sometimes you won't realize the clock stopped because they'll switch to a TV commercial.