You start with a liquid containing some kind of sugar, add yeast, and wait. The yeast digests the sugar, giving off alcohol in the process. The sugar in beer comes mostly from a grain called barley. With wine, the sugar usually comes from grapes. Either way, when the yeast finishes its work, the sugar has been turned into alcohol–which stays–and carbon dioxide, which bubbles to the surface. Since wine starts out with more sugar than beer does, it ends up with more alcohol. If you make beer or wine at home, the trick is knowing just how much sugar to start out with in order to get the alcohol content you want to end up with. However, if the alcohol gets too strong, it’ll poison the yeast, stopping the fermentation altogether.