Four forces make baked goods rise:

• Chemical leavening - baking soda (a base) reacts with an acid to produce CO2 bubbles. Baking powder packs both the base and the acid into one package.
• Biological leavening - yeast ferments sugars and produces CO2. It is slower but builds more complex flavors.
• Mechanical leavening - whipping air into eggs or butter. Angel food cake relies on trapped air expanding in the oven.
• Steam - water in the dough turns to vapor and expands to about 1,600 times its liquid volume. Choux pastry gets its dramatic hollow lift mostly from steam.

Most baked goods use a combination. Croissants use yeast, steam, and mechanical layering all at once. Knowing which leavener dominates tells you what can go wrong.