Out Of Bounds
Anytime a player in possession of the ball goes out of bounds the play clock stops. According to the NFL Rule Book, the clock is re-started once the officials spot the ball at the appropriate yard line on the field except during the last two minutes of the first half or the last five minutes of the fourth quarter. During these times the clock doesn't start again until the next snap. The offense must run the play before the play clock runs to zero, however.
If a player in possession of the ball has his forward progress stopped in bounds and is then pushed out of bounds, the officials may mark the player down in bounds at the point of forward progress and signal for the clock to continue running.
Also, if a player's momentum is going backwards when he runs or is pushed out of bounds, the play clock will continue running. A player's momentum has to be going forward as he goes out of bounds in order for the play clock to stop.