Angel Food Cakes

How does the angel food cake achieve its airy volume and its fine, even, tender texture without any baking powder or fat? This one-bowl cake, which is simple to assemble, owes its success to the proper whipping of egg whites. When the cake is in the oven, some volume builds from the air cells you have whipped into the whites. But most of the volume is produced by the steam which evaporates from the liquid in the egg whites and passes through the air cells of the egg whites, expanding them.