The triathlon is the ultimate test of stamina, strength and skill - a 1500m swim, 40km bike ride and 10km run. Here's our guide to this gut-busting sport...
When the gun goes, it's a sprint to the water and a desperate fight to get to the front first. Athletes wear wetsuits but with the adrenaline pumping, they'll barely notice the cold.
An early lead is important. If you're at the front, you'll be in calm, clear water. But if you're further back, you'll be lost in a sea of limbs and in danger of having your goggles ripped off.
The swimmers concentrate on three things - keeping their stroke long and efficient, a relaxed, steady rhythm, and regular glances to make sure they're not straying off-course.
As the competitors complete the swim leg, they sprint to their bikes and start to pull off their swimming gear. The rush of blood from upper body to legs can result in dizziness.
The racers grab their bikes and run with them to the start of the bike course. Helmets must be on before they exit and shoes will already be clipped to the bike's pedals to save time.
The bike's set-up maximises speed. The frame is lightweight carbon and the handlebars shaped to cut wind resistance. Clothing and helmets are skintight to further reduce drag.
Riders try to slip-stream behind rivals to save energy during the longest leg of the race. They try to reduce fatigue in their quadriceps by staying seated in the saddle, even on climbs.
The athletes expend huge amounts of energy and lose substantial water through sweat as they race. Each will carry at least one bottle of isotonic energy drink on their bike.
Leaving their stiff-soled shoes on the bike, the athletes re-enter the transition area, ditch the bikes and slip on running shoes. The shoes have pre-tied elastic laces to save time.
Exhausted, they now have to dig deep. Although it lasts just over half an hour, the running is where the race is won. A great runner usually beats a great swimmer to the finish.
After about one hour 45 minutes for men, and two hours for women, the race is over. The distance covered and the variety in conditions make it very hard to predict a winner.
Lying on the ground might be what the athletes feel like doing but it only makes things worse. A slow jog, warm, dry clothes and immediate intake of carbohydrates are all essential.