Fermentation is the key word, mimi... The yeast (Saccharomyces cerevisiae), is a single celled microscopic fungus that converts the complex carbohydrates in the bread recipe's flour into simple sugars that it feeds on. With an almost instant action it starts to release carbon dioxide and alcohol, all very important by-products in bread-making.The alcohol, by the way, from the fermentation process evaporates off during baking. When protein, starch, and fat molecules break down into their building blocks during fermentation, it breaks down large molecules into smaller, flavorful ones.-- proteins into amino acids, starches into sugars, or fats into free fatty acids. Yeast generates carbon dioxide which makes the dough rise. (Some thanks to Baking 101)
Same process makes the carbon dioxide bubbles in beer and champagne...