Until then I, like most home bakers, had been using commercial yeast to leaven my bread. Flatbread, essentially flour and water cooked on a hot stone, dates to the Paleolithic era, but it was the ancient Egyptians who discovered that if you added some yeast—and there was plenty of that around, since they were accomplished brewers—the dough would rise into a loaf. Some 6,000 years would pass before Louis Pasteur would unlock the secret why, and it's really quite simple and beautiful: Living yeast, when fed sugar, produces carbon dioxide and alcohol, the process we call fermentation. Meanwhile, flour, in the presence of water, produces those long, tangled protein chains called gluten. Kneading stretches the coils out, aligning them side by side so that they can bond, forming the strong elastic network that enables the dough to stretch and capture the carbon dioxide gases emitted by the yeast. What wonderful choreography! Subtract any one dancer, and the ballet falls apart.