Sourdough is the oldest form of leavened bread, discovered accidentally in Ancient Egypt when wild yeast drifted into a dough mixture and began to ferment. Unlike commercial bread that uses isolated yeast, sourdough relies on a 'starter'—a symbiotic culture of wild yeast and lactic acid bacteria. The yeast produces carbon dioxide to lift the bread, while the bacteria (mostly *Lactobacillus*) produce lactic and acetic acids, giving the bread its characteristic sour tang and acting as a natural preservative. This fermentation process also breaks down gluten and phytic acid, making sourdough easier for some people to digest than modern processed breads. Some bakeries in San Francisco and Europe claim to use starters that have been kept alive and fed daily for over 100 years.