The substance that makes bread rise is yeast, a type of fungus that ferments when mixed with sugar and moisture. Yeast consumes the sugar and produces carbon dioxide gas and alcohol as byproducts during fermentation. The carbon dioxide gas gets trapped in the dough's gluten structure, causing it to expand and rise as it bakes. This process gives bread its light and airy texture. There are various types of yeast used in baking, such as active dry yeast, instant yeast, and fresh yeast. Properly activated yeast is crucial for achieving the desired rise in bread and other baked goods.