Final answer:
Moisture and heat are the two things that activate leavening agents in baking. Yeast begins fermentation when combined with moisture, and this process is accelerated by heat. Chemical leavening agents like baking soda require an acid and heat to produce carbon dioxide gas that makes dough rise.
Step-by-step explanation:
The two things that activate the leavening agent in baking are moisture and heat. When yeast, a common leavening agent, is mixed with the moisture in the dough, it begins the process of fermentation, producing carbon dioxide. As the dough is heated during baking, the carbon dioxide gas expands, further leavening the bread and creating the familiar airy texture.
In the case of a chemical leavening agent like baking soda, the activation requires the presence of an acid. When baking soda (a base) comes into contact with an acid like vinegar or lemon juice, a chemical reaction occurs, producing carbon dioxide gas. This reaction is accelerated by the heat of the oven, contributing to the rising of the dough or batter.