Since there is no stopping the clock in a soccer game, the ref keeps track of how much time is wasted during each half by players faking injuries, chasing down balls out of bounds, celebrating after scoring, or otherwise not playing the game. That wasted time is added onto the end of each half (called stoppage time) so that there will be 45 minutes of playing time per half. Also allows teams like Argentina and Portugal to make dramatic plays to save their own collective azzes.
Replies