Swimming in 2009: False start

Four strokes, one clock, and the oldest question in the water: who reached the wall first. 209 national federations under World Aquatics World Aquatics (founded 1908 as FINA) On the programme at every Games since 1896 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.

What grips audiences is the brutal clarity. Every Olympic final is a settled argument, eight bodies, one clock, no ambiguity. The drama compresses into 21 seconds for a 50m sprint or unfolds across 15 lung-searing minutes in the 1,500m. Swimming also produces the Olympics' defining serial champions: Michael Phelps, Mark Spitz, Katie Ledecky. Because the sport peaks every four years on the largest stage on earth, careers are measured in Olympic cycles, and a single touch-out, Phelps beating Milorad Cavic by one hundredth in Beijing, can define a legacy. Few sports pair such physiological extremity with such simple, unforgiving scoring.