When the dough hits the hot oven, it can puff up to as much as a third of its size in a matter of a few minutes. This oven spring is a good indicator of the crumb of your bread: more oven spring means a light and airy interior and little oven spring indicates a dense, compact crumb.
More on how oven spring works after the jump!
During those first few minutes as the dough starts to heat up, the yeast go into a feeding frenzy, converting the sugars in the dough into carbon dioxide and alcohol. Water in the dough is simultaneously turning into steam and pushing its way out of the dough.
This steam and the carbon dioxide from the yeast work together to inflate the dough like a balloon. Once the internal temperature of the bread reaches about 120-degrees, the yeast stop reproducing and the crust begins to solidify, curtailing the rise of the dough.
(Image Credit: Emma Christensen for the Kitchn)