I taught a science lesson to my son's fourth grade class yesterday on different types of leavening agents, using pancakes as my visual (and edible) aid. I used pancakes made with a sourdough starter to illustrate a biological leavener (wild yeast) and the reaction of baking soda, a chemical leavener, when combined with the acids produced by the yeast in the starter.
We compared that to whole wheat pancakes made with another chemical leavener, baking powder. You can see the difference in the bubbles between the baking powder reaction in the whole wheat pancake above and the sourdough/baking soda pancake below.
We also talked about the physical leavening power of the beaten egg whites and steam created by the liquid in the batter and the heat of the griddle.