A byproduct of yeast eating sugars is carbon dioxide. The bread dough, being in a fluid state, but stiffened by the glutens does not let the gas release easily, instead causing pockets or bubbles to...
It is the action of the yeast that causes breads to rise. Yeast as it grows produces carbon dioxide this is trapped in the bread dough and causes the dough to rise. Leavening with yeast is a process...