The chemical action and growth of yeast that caused the dough to rise then became understandable. They are egg-shaped cells that can only be seen with a microscope. Cerevisiae is the Latin word for brewer. A very long name for such a tiny organism. It is a very strong strain that is capable of fermentation or causing bread dough to rise.
Yeast cells digest food to obtain energy for growth.
The process, alcoholic fermentation, produces useful end products, carbon dioxide and ethyl alcohol, which are released by the yeast cells into the surrounding liquid. This is how alcoholic drinks are produced from starch containing flours, i.e., barley flour for making beer and wheat, corn or other grains for making whiskey. Fermentation occurs naturally in nature. For instance, many berries break open in late fall when they are over-ripe and full of sugar. Natural yeast from the air, so tiny they cannot be seen, lodge on the surface of these berries, which then become fermented and alcoholic.