Swimming is an individual or team racing sport that requires the use of one's entire body to move through water. The sport takes place in pools or open water. Competitive swimming is one of the most popular Olympic sports, with varied distance events in butterfly, backstroke, breaststroke, freestyle, and individual medley. In addition to these individual events, four swimmers can take part in either a freestyle or medley relay. A medley relay consists of four swimmers who will each swim a different stroke, ordered as backstroke, breaststroke, butterfly, and freestyle. Men and women now compete in 17 events each at the Olympics, involving four different strokes across a range of distances. Freestyle races cover 50m, 100m, 200m, 400m, 800m (women only), 1500m (men only) and 10km. The butterfly, backstroke and breaststroke race each cover 100m and 200m. All four strokes are used in the 200m and 400m individual medley events. The 4 x 100m freestyle, 4 x 200m freestyle and 4 x 100m medley relays complete the program. Each race has a maximum of eight swimmers. Preliminary heats in the 50m, 100m and 200m lead to semi-finals and finals based on the fastest times. In relays and individual events of 400m or more, the eight fastest finishers in the preliminaries advance directly to the finals.