Water polo is one of the most physically demanding sports of the Olympics as it forces competitors to constantly swim while also having the strength to fire a ball toward a goal, often over defenders swimming in front of them.
There are four quarters in a game, each lasting eight minutes. Each team has 30 seconds of starting an attack to take a shot at the goal or the possession changes hands.
The goalkeeper can put both hands on the ball, but all the other competitors can only use one hand. To help them grip the wet ball, it is made of a special type of rubber and has a grooved surface.
The playing area is about 98 feet by 65 feet, and must be at least 6 1/2 feet deep, so players are constantly treading water. Add to that, they have to lift their upper bodies out of the water to pass or shoot over their opponents. Due to all this exertion, rolling substitutions are allowed.
Fouls are frequent and can result in players being held in an exclusion zone (think of a penalty box in hockey) for 20 seconds or simply lead to a change of possession. That's for the contact the referee can see. A lot of the contact happens underwater.
There will be 12 teams on the men’s side and 10 for the women. After pool play, the top eight teams in each will advance to the quarterfinals. The semifinal winners compete for the gold while the losers will battle for the bronze.