The main ingredients of bread are yeast, wheat flour, sweetener, salt and a liquid, such as water. The yeast is largely responsible for making the bread dough rise.
When wheat flour and liquid are mixed, gluten forms. As the dough is kneaded, the gluten stretches. This creates the structure of the bread, trapping the carbon dioxide gas that the yeast releases. This causes the bread to rise and expand.
Yeast consumes the sweetener ? often sugar or honey ? and creates carbon dioxide and alcohol. Salt helps limit how fast the yeast feeds, which keeps the rising slow and steady.