Final answer:
The primary leavening agent in eclair paste is steam. During baking, the moisture in the dough turns into steam, puffing up the paste, with eggs providing stability.
Step-by-step explanation:
The primary leavening agent in eclair paste is steam. When making éclair paste, also known as pâte à choux, water and butter are heated together and then flour is added to form a dough.
During the baking process, the high moisture content in the dough turns into steam, which puffs up the paste. The eggs in the recipe help to stabilize and enrich the dough, but the expansion comes primarily from the steam generated in the heated oven.