Why does bread rise when baking?

Answer: Yeast produces CO2 gas

Heat expands the dough — Wrong. Heat alone doesn't make bread rise. While heat does cause some expansion, the main rising happens before baking through a biological process. The heat actually sets the structure after yeast has done its work.

Yeast produces CO2 gas ✓ — Correct! Yeast is a living microorganism that eats sugars in the dough and produces carbon dioxide gas as waste. These CO2 bubbles get trapped in the gluten network, making the dough expand. Heat from baking makes the gas expand more and kills the yeast, setting the fluffy structure!

Air bubbles get bigger — Wrong. Air bubbles don't spontaneously get bigger. The bubbles are filled with carbon dioxide produced by yeast through fermentation. It's this biological process of yeast consuming sugars and releasing CO2 that creates the gas bubbles.