Water polo is just one of several water-based sports that will be played in Tokyo at the upcoming Summer Olympics. The football-active sport requires athletes to be strong swimmers, handle a ball well and work together as a team. However, there is one rule that makes water polo even more difficult than it looks on the surface: Players can never touch the bottom of the pool.

According to Swim England, the UK’s governing body for swimming, “players must not touch the bottom of the pool and must tread water all the time. Water polo players use a movement called egg baking, which is more efficient than normal treading water action.” movement is similar to the circular kick used in the chest stroke, except when the legs are changed instead of kicking at once. Not only is it a more efficient movement for long periods in the water, but it is also much more stable, which is perfect for water polo players who need to use their upper bodies to pass the ball and score.

If a player does not stay afloat during gameplay and does not touch the bottom of the pot, it counts as a minor mistake, according to USA Water Polo. When a minor offense like this is committed, the team that does not commit the offense (or who was committed against, in the case of other types of minor offenses) restarts the game by getting a free throw. Other errors in this category include things like keeping the ball under water to keep it away from an opponent, delaying the game, touching the ball with two hands, or a goalkeeper moving past the center line.

Water polo can be a fast and intense game, which makes it a lot of fun to watch. But make no mistake, it also requires a lot of strength and conditioning from the athletes – as if you needed another reason to be impressed.

To learn more about all the Olympic hopefuls, visit TeamUSA.org. Watch the Olympics in Tokyo this summer on NBC.