Yeast is what makes bread and other baked goods fluffy and light Yeast is actually a fungus that thrives on sugar. Then, when it is mixed with the right baking ingredients, the yeast will create carbon dioxide that makes the dough expand and rise. It also adds flavor. You can easily cultivate your own yeast with just two ingredients – flour and water. The fungus that creates yeast is everywhere – on your skin, in the air, and in flour. With just a little time and the right 'feeding' schedule, you can cultivate your own yeast that we know as sourdough starter.