Swimming in : Starts

Freestyle, backstroke, breaststroke, butterfly Michael Phelps: 28 Olympic medals, 23 gold Swimming is the purest test in sport of a human moving through water, a contest decided by hundredths of a second between an athlete, the clock, and the resistance of a medium roughly 800 times denser than air. Races are contested over four strokes (freestyle, backstroke, breaststroke and butterfly) and distances from an explosive 50-metre dash to the 1,500-metre 'mile', in pools 50 metres long (long course) or 25 metres (short course). Unlike most sports, there is no ball, no opponent to tackle, no referee's judgment on the outcome: an electronic touchpad records the truth to the thousandth of a second, and the wall never lies.

Swimming is unique in how much of the race happens where fans cannot see. The fastest swimmers win underwater, in the streamlined dolphin-kick phase off each wall, exploiting the 15-metre limit that turned the breakout into a discipline of its own. Technique, the 'catch', distance-per-stroke, streamline, turns, matters as much as raw power, and marginal hydrodynamic gains reshape the sport, as the banned 2009 polyurethane super-suits proved when 43 world records fell in a single week. It is also among the most globally practised human activities, a survival skill before it is a sport, governed by World Aquatics across 209 national federations and contested at every modern Olympics since 1896.

At major meets each event is run over multiple rounds. Swimmers first race in heats (preliminaries); the fastest advance to semi-finals for 50m to 200m events and then an eight-lane final, while 400m-and-longer events often go straight from heats to a timed final. Lane assignments reward the fastest qualifiers with the centre lanes (4 and 5), where the water is calmest. Relays add a team dimension: four swimmers each complete an equal leg, with later swimmers timing their dive off the block to the incoming teammate's touch.