The tradition of the Olympic flame is rooted in Greek sporting heritage dating back thousands of years but new technology keeps the fire burning whatever the elements - or modern-day protester - can throw at it.
This week a torch was briefly extinguished in Paris to keep it from protesters opposing China's policy in Tibet.
The Olympic flame, though, did not go out.
Two lamps, lit from the original flame of Olympia, accompany the relay and help light thousands of torches carried by bearers who include celebrities, politicians and athletes on its 137,000 kilometre (85,000 miles) journey.
The Beijing torch burns on environmentally friendly propane gas and its flame can last up to 15 minutes. Every torchbearer has a separate torch that they can buy to keep as a souvenir at the end of their run.
"(The gas) is composed of carbon and hydrogen. No material, except carbon dioxide and water remain after the burning, eliminating any risk of pollution," the Beijing Games organizing committee explains on its website.
The burning system itself was designed by China Aerospace Science and Industry Corporation.
The Sydney 2000 torch used a mix of butane and propane for a rich flame effect that could also burn underwater at the Great Barrier Reef.
The 72-cm tall Beijing torch, weighing just under one kilo, is always accompanied by several vehicles and security guards on foot, while one of two portable backup lamps, follows in a van.
"The flame will often go out during the relay, due to weather conditions or problems with the gas," said former Athens 2004 Games official Pierre Kosmidis, who accompanied the flame on its long international leg four years ago.
"When travelling by plane, the flame is kept inside two small portable lamps. Upon arrival the lamps are kept apart, one following the torch in a van in case it goes out and one kept at a separate location as backup," Kosmidis said.
Security guards running with the torchbearers turn off the torch at the end of the daily relays, before the torch is lit again from the lamps for the relay the next day.
The torch, introduced for the first time at the 1936 Berlin Olympics, is still an engineering work in progress as technicians seek the perfect flame.
There was no relay in the ancient Games in Olympia, although there were several burning flames at those Games as well as flame races at the Panathenian Games to honour deities including Zeus and Prometheus, who, legend has it, stole fire from the gods and brought wisdom and knowledge to humankind.
The torch relay was introduced in 1936 and the first torch, constructed by the Krupp steel and munitions company, crucial to Adolf Hitler's war preparations, used solid fuel skewered on a needle inside the torch to burn the flame after it was lit from the sun's rays via a parabolic mirror in ancient Olympia.
The torch's general features have stayed more or less the same since then. It must be light enough to carry and must have a stable and visible burning system that will not singe the runner's hair or hands and will allow for media coverage.
That has not always been the case though.
The torch conceived for the 1968 Mexico City Games may arguably have been the most stylish one, but sparks flew during runs and the torch itself became too hot to handle, forcing organizers to quickly replace it with a modified version.
The Barcelona 1992 Games torch was also problematic, its plastic parts melting in the hands of several runners.
Torches in recent Games, including Beijing, have switched to using gas cartridges that allow greater control of the flame's size and colour and are safer for runners.Report Typo/Error
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