Hydrogen sulfide—the colorless gas that smells like rotten eggs—is a natural by-product of fermentation (caused by the reaction of sulfur contact with the dead yeast or lees after primary alcoholic fermentation). It usually dissipates gradually throughout the winemaking process and is easy to remove from a young wine through simple racking and aeration. It has a rather nasty side effect, however: if hydrogen sulfide isn't completely aired out of the wine through simple racking and aeration, it will combine with the alcohol to create ethyl mercaptan, a sulfur-based organic compound that emits some rather unpleasant odors ranging from skunk to garlic, burnt rubber, and onion.