Yeast is a tiny living organism, a type of fungus so small you need a microscope to see individual cells. Despite their size, yeast cells have an extraordinary ability: they eat sugar and produce carbon dioxide gas and alcohol as waste products. This process, called fermentation, makes yeast one of humanity's most useful microscopic partners.

When you mix yeast into bread dough, the cells consume sugars in the flour and release thousands of tiny carbon dioxide bubbles. These bubbles get trapped in the sticky dough, causing it to rise and become light and fluffy. Without yeast, bread would be flat and dense, more like a cracker.

Yeast exists naturally all around us: on grape skins, in the air, and on grain kernels. The white dusty coating you see on fresh grapes is partly wild yeast. When you buy a packet of yeast at the store, you're getting millions of dormant cells waiting to wake up and start eating and reproducing when you add them to warm water.