By way of a viewer question, SciShow host Michael Aranda talked about the history of sourdough bread and the thing that makes it different from other breads. Aranda also explained how to go about creating a sourdough-starter made up of wild yeasts using a natural fermentation process.
But before grocery stores, people had to culture their own wild yeasts, keeping them alive in something called a starter. You can make sourdough starter by mixing some flour and water and letting it sit out, because the air around us is /full/ of microbes like yeast and bacteria. After some time, different species will make their home in the goop, and keep multiplying as long as you keep feeding them. Microorganisms like yeast and bacteria break down organic compounds in a process called fermentation – producing other chemicals like gases or alcohols. So when yeasts digest the sugars in flour, they make bubbles of carbon dioxide gas, which make bread dough rise.