Here's how submarines handle these important functions. When we breathe, our bodies consume oxygen and convert it to carbon dioxide and moisture. Since carbon dioxide is poisonous, life support systems must remove the carbon dioxide, put oxygen back into the air and remove the moisture that the crew exhales in each breath. There are computerized sensors on board submarines that constantly measure the amount of oxygen, carbon dioxide and moisture in the air. To maintain oxygen at 21 percent of the volume of air, submarines have a special machine called an oxygen generator that removes oxygen molecules from seawater and pumps them into the ship's air supply. This provides an unending supply of oxygen for the crew. As a backup, submarines also have pure oxygen in pressurized tanks.

Carbon dioxide is removed from the air by a special filtering device called a scrubber. The scrubber traps carbon dioxide by a chemical reaction and reduces the amount of carbon dioxide from 4.5 percent in exhaled air to .04 percent.

Finally, moisture is removed from the air by a device called a dehumidifier. It works something like the air conditioner on your family's car. In a submarine, if moisture wasn't removed, it could be foggy or even rain inside!

Two hundred years ago, a British poet wrote, "Water, water every where, nor any drop to drink." The same can be said for the crew of a submarine. Submarines need a lot of fresh water -- as much as 40,000 gallons per day for drinking, cooking, bathing and for cooling computers and other equipment. Modern submarines have specially designed systems to produce fresh water from seawater. This is done by a process called distillation. First, seawater is heated to its boiling point. The pure water evaporates, leaving the salt and impurities behind. Then the water vapor is cooled and collected in large tanks for drinking and other purposes.

The water temperature of the ocean surrounding submarines is typically 39 degrees Fahrenheit (4 degrees Celsius). The metal hull of a submarine is a good conductor of heat, so without an internal heating system, the submarine's internal temperature would be a frosty 39 degrees too! Submarines are heated electrically, with the power supplied by onboard generators or backup batteries.

The Archimedes Principle Although he lived more than 2,000 years ago, Archimedes, a Greek philosopher and mathematician (287-212 B.C.) is still regarded as one of the greatest physical scientists of all time. Perhaps you have heard the story of the King's crown. It is told that, while talking a bath, he discovered a method for figuring out whether the crown had been made of pure gold or a cheaper metal. Archimedes was so excited about his discovery, he ran straight from the tub through the streets shouting, "Eureka!" We now call his discovery Archimedes' Principle, which states that an object placed in a fluid is buoyed upward by a force equal in weight to the fluid displaced (pushed out of the way) by the object.