On the orbiting Mir space station, urine and sweat are the basic staples of life. In recent weeks, as the cosmonauts aboard the 11-year-old station have undertaken risky repairs to repair the ailing ship, they have also tinkered less dramatically to keep their life-support systems working.
While some of Mir's systems require complex computer commands and high-tech wizardry, the ship's air and water systems need the fuel of just urine and sweat.
"We have something to be proud of," said Mr Yevgeny Zaitsev, the official at the Energiya rocket firm who designed Mir's life-support systems. "This complex is unique - there is no other like it in the world."
The crew, consisting of two Russians and one American, has recently been working to restore the main Elektron oxygen generating system, a metre-long cylinder that transforms a cosmonaut's urine into oxygen.
The chemistry is straightforward, recycling urine into water and then stripping out the hydrogen from the water, which consists of hydrogen and oxygen, or H 2O. With the cosmonaut's daily output of 1.2 litres of urine, the Elektron makes 0.86 kg of oxygen - the exact amount needed daily, Mr Zaitsev said.
In recent weeks, the Mir crew has relied on an older back-up Elektron sent up in 1993. "The Elektron should last for 320 to 340 working days, but we keep using the systems until they fail," he said.
The crew is also testing an Elektron sent up earlier this year that only worked for a few weeks before breaking down.
As another back-up, cosmonauts sometimes also use oxygen "candles", cannisters the size of narrow books which produce oxygen when burned in a special onboard device.
"Each cosmonaut reacts differently to the air on Mir. Some say it is just like fresh air, others say it smells differently," said Mr Zaitsev, who is in frequent communication with the crew from Mission Control in Korolyov outside Moscow.
For drinking water, the cosmonauts rely mostly on their sweat, which floats weightlessly round the station before gathering into another water purifier.
"A person produces a litre and a half of sweat a day," Mr Zaitsev said. "From this we produce distilled water, but this water is not tasty, so we add salts so it tastes like the water you know."
Over the past 11 years with Mir in orbit, scientists have learned that a cosmonaut needs 2.7 litres of water a day. Of that 1.5 comes from sweat recycling, 0.6 from moisture in food, and the rest from fresh supplies sent up from Earth.
Other human waste products are not recycled. Solid waste is gathered and eventually carted away on the Progress cargo ship. Another on-board filter strips out harmful elements from human gases, dumping them into space.
The carbon dioxide cosmonauts breathe out goes into the Vozdukh system, which pipes carbon dioxide into space while keeping breathable air on board.
During a spacewalk planned for early Saturday, cosmonauts plan to start installing another Vozdukh carbon dioxide removal system so they will have a back-up, just in case, officials say.