What makes bread rise?

There are various techniques and ingredients used in the bread making process to make it rise. Yeast can be used in the dough which will activate and the dough will rise over a number of hours (or even days) depending on the type of bread you are making. Another method is natural fermentation ie sourdough. Sourdough bread rises by using a "pre-ferment" or sourdough starter when mixing the dough. Typically sourdough requires a longer rise time for the development of the air bubbles and the sour taste by which the name comes from.