Fermentation has been used by mankind for thousands of years for brewing beer, fermenting wine and raising bread. The products of the fermentation of sugar by baker's yeast Saccharomyces cerevisiae (a fungus) are ethyl alcohol and carbon dioxide. Carbon dioxide causes bread to rise and gives effervescent drinks their bubbles. This action of yeast on sugar is used to 'carbonate' beverages, as in the addition of bubbles to champagne). [Note: In response to many questions I have received, here is a discussion of the small amount of ethyl alcohol which results in this root beer .] We will set up a fermentation in a closed system and capture the generated carbon dioxide to carbonate root beer. You may of course adjust the quantities of sugar and/or extract to taste. You should be able to find root beer extract at your local supermarket.  Hires and A&W have a long history of makiing root beer extract.  I find Zatarain's extract especially delicious, but your definition of root beer may include a different assortment of flavors.  If you can't find it, Zatarain's, a product of New Orleans can be ordered on the web.  Other flavors can be substituted for the root beer extract.  Try using a tablespoon of vanilla instead of the extract for a cream soda, and grated ginger and lemon for ginger ale . IMPORTANT SAFETY CAUTION: As you follow the following recipe, be sure to refrigerate these bottle-fermented soft drinks as soon as the bottle feels hard .  Especially in