Swimming in 15: Butterfly rules

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.

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.

The objective is elemental: cover the race distance in the prescribed stroke faster than everyone else, touching the wall first. A competitive swimming contest is a series of individual and relay races (events) run in a pool divided into 8 to 10 lanes. Each event specifies a stroke and a distance, for example the 100m butterfly or 400m individual medley, and swimmers are seeded by entry time. The winner is simply the athlete whose hand or body strikes the electronic touchpad first, timed automatically to a hundredth of a second.