Factbox: Key facts about water polo at the Tokyo 2020 Games

(Reuters) - The 2020 Tokyo Olympics includes 33 sports. Water polo is one of five aquatic events.

Here are some key facts about Olympic water polo:

Introduced: Water polo debuted at the modern Olympics in 1900 with a men's tournament. The women's competition was added a century later for the 2000 Games in Sydney.

Events: Men play in a 12-team tournament. Women compete in a 10-team event, up from eight at the 2016 Rio Olympics.

Scoring: Matches consist of four eight-minute periods with the team scoring the most goals winning.

Players cannot touch the sides or bottom of the pool so have to tread water continuously. Teams must shoot within 30 seconds, counted on a shot clock, or they lose possession.

Medal winners at the Rio 2016 Games: Serbia won the men's gold, Croatia silver and Italy bronze. The United States retained the women's title they won at London 2012, with Italy and Russia taking silver and bronze respectively.

Hungary are well-known for their successful water polo programme, with the men's team winning gold nine times.

Sources: FINA, the International Olympic Committee (IOC)

(Graphics: Water polo at the Tokyo 2020 Games - https://graphics.reuters.com/OLYMPICS-2020-WATERPOLO/0100B5D73H4/WATERPOLO.jpg)

(Reporting by Amy Tennery; Editing by Ken Ferris)