Why does bread rise?
The short answer
Yeast breaks down sugars and produces carbon dioxide. The dough's network traps the bubbles and expands.
How it works
Too much heat harms yeast, while low temperatures slow it. In the oven, gas expands, yeast stops working, and the dough structure sets.
A familiar example
The same dough can develop different aromas and bubble sizes depending on fermentation time and temperature.