Carbon dioxide is a chemical compound containing one carbon atom and two oxygen atoms per molecule. At normal temperatures it exists as a gas, and is present as a trace element in the atmosphere. Plants absorb carbon dioxide (along with light and water), using a process called photosynthesis to produce energy for themselves. Oxygen is then released as a waste product of this process. Oxygen is used by humans, elves, merfolk, and animals as part of their respiratory process. Carbon dioxide is released as a waste product of this process. It may also be released as a byproduct of fermentation (which can be used to produce alcohol and vinegar, and also occurs in baking), as well as a byproduct of combustion. The gas may also be found naturally occurring in some mineral springs (most famously, Lymon Mists).
In LY 806, naturally carbonated spring water came to be used as an ingredient in soft drinks. Since that time, methods of artificially carbonating water have been developed, most commonly by suspending containers of water over vats used in fermenting alcohol, which has led to some spiritmakers also selling bottled carbonated water (plain or flavored as soft drinks). In 914, Anja Frontrun introduced a new method of pressurizing and storing carbon dioxide, using piston pumps, which has led also to new methods of dispensing both beer and soft drinks. In 916, some scientists have begun using piston pumps to compress carbon dioxide and produce "dry ice," which may be used for purposes of refrigeration.