It is regarded as one of the toughest sports at the Olympics, and the only ball sport at the games played in the water.
Water polo is one of the longest running sports at the games and the men’s competition has been part of the Olympics since 1900, with the women’s tournament only starting at Sydney 2000.
The sport is played in a pool that is two-metres, around 6.5ft, deep and a player’s feet must not touch the bottom.
Both teams have seven players in the pool at any time, and when they have the ball a team’s players must be constantly swimming and have 30 seconds to shoot the ball.
The sport is notorious for its physical play, and contact is allowed at any time, even when an athlete does not have the ball.
But splashing water in an opponent’s face, striking them or interfering with a free throw are all considered fouls.
A match has four, eight-minute quarters and the highest score wins, with a penalty shootout breaking any ties.
Hungary is the most successful country in water polo in Olympic history with 15 medals, including nine golds and three in a row in 2000, 2004 and 2008.
Great Britain won the first ever gold medal and has won four in total, but it has not won gold since 1928, and has not finished higher than fourth since 1928.
The American men’s team has won six medals in its history, but has never claimed gold.
But the American women’s team has medalled in every Olympics since its introduction and has won the last two gold medals.
The US women won their first match in Tokyo against China, but then lost to Hungary, its first defeat at the Olympics since 2008.
They are still expected to reach the knockout rounds of the competition.
The men have so far beaten Japan and South Africa.
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