Bread rises because of gas trapped in dough. The usual domestic method uses yeast, a single-celled fungus. Mixed with flour and water, yeast eats sugars, some already in the flour, more released by enzymes as starch breaks down. It releases carbon dioxide and alcohol. Gluten, a network of proteins that forms when wheat flour is hydrated and worked, stretches around those bubbles instead of letting them pop. Heat in the oven expands the gas, sets the proteins and starches, and drives off much of the alcohol. What remains is a sponge you can slice.

Sourdough relies on wild yeasts and lactic bacteria living in a starter. The bacteria add acid and flavor and help the dough keep. Some breads use baking soda or baking powder instead, chemical sources of gas that do not need hours of fermentation. Steamed buns, flatbreads, and crackers show that rise is optional. Culture decided that a tall loaf was worth the wait.

Flour type matters. Hard wheat has more gluten and builds higher loaves. Rye has less and a different chemistry, so rye breads are denser unless mixed with wheat. Salt slows yeast and tightens gluten. Sugar feeds yeast but in excess can slow it. A kitchen loaf is agriculture, microbiology, and physics in one pan: grain, microbes, water, and heat agreeing, briefly, to hold the shape of air.
