Every recipe that rises is doing one specific thing: trapping gas inside a batter or dough faster than the gas can escape. A leavener doesn't lift anything on its own — it produces or releases gas, and something else in the mixture has to be strong enough to hold that gas until the structure sets around it. Skip that second part and the same batter can puff in the oven and sink again before it even cools, no matter how good the leavener was.
Only three things ever make the gas
Every leavening method on earth reduces to one of three sources. Biological: yeast, a living organism, eats sugar and releases carbon dioxide as it ferments, slow and steady, the reaction behind risen bread. Chemical: baking soda and baking powder react with moisture and, in soda's case, an acid, to release carbon dioxide fast, in minutes rather than hours. Physical: water turns to steam at 212°F (100°C) inside a hot oven, and that expanding vapor is most of what lifts a choux puff or a popover on its own; whipped egg whites or creamed butter work the same physical trick, holding air inside a foam that expands when heated. Nothing else makes gas. Whatever a recipe calls a leavener is doing one of these three things.
Baking soda needs an acid; baking powder carries its own
Baking soda is sodium bicarbonate, and on its own it's just an alkaline powder sitting in a bowl. It only releases carbon dioxide once it meets both moisture and an acid — buttermilk, brown sugar, yogurt, lemon juice, cocoa powder. Without enough acid in the recipe, some of the soda never reacts, and what's left behind tastes faintly metallic and soapy, not sour.
Baking powder solves that by packing its own acid into the can. It's baking soda mixed with a powdered acid, usually cream of tartar or sodium aluminum sulfate, plus a bit of cornstarch to keep the two apart until they're wet. That's why baking powder works in a plain milk-and-flour batter with no added acid at all. The acid is already in the tin, waiting for water.
Double-acting powder is why batter can sit before baking
Most baking powder sold today is double-acting, meaning it carries two different acids that fire at two different points. One dissolves and reacts the moment it hits liquid, at room temperature, on the counter. The second stays inert until it's heated past roughly 120°F (49°C), which only happens once the batter is actually in the oven. That's the whole reason cake batter can sit in a bowl for a few minutes without deflating — most of its gas hasn't been made yet. Single-acting powder and plain baking soda don't get that grace period. Mixed with liquid, their gas starts leaving whether the oven is ready or not.
Structure is what turns gas into rise, not the leavener
Gas alone doesn't make anything taller. A glass of soda water is full of carbon dioxide and it doesn't rise an inch, because nothing around those bubbles holds them in place as they grow. Batter and dough do that job with a stretchy protein network — gluten in a flour-based batter, coagulating egg proteins in a soufflé or sponge — that traps each bubble as a leavener produces it and stretches along with it, a skin expanding around the gas going in.
That network only becomes permanent once heat sets it. Flour and egg proteins are soft going into the oven and firm up as they cook, locking the risen shape in place. If gas arrives faster than the structure can be built — too much leavener, too little flour or egg, an underbaked center pulled out too soon — the bubbles keep merging and growing until they tear through the weak spots, or the whole structure just gives out before it's set. That's a soufflé falling, a quick bread with a sunken middle, a pancake that puffed and flattened again. The leavener did its job completely. The structure just wasn't ready to hold what it made.