Why does pancake batter become fluffy instead of staying flat?

A spoonful of pancake batter spreads across a pan. Soon bubbles appear, the edges become firmer, and a soft pancake takes shape. For a baking-powder pancake, two processes have to work together: gas makes spaces inside the batter, and heat makes the surrounding material firm enough to keep them.

Baking powder contains ingredients that react when moistened to release carbon dioxide gas. Many types release additional gas as they warm. The gas collects in bubbles already present or forming in the mixture. Warming also expands trapped gas, while some water becomes vapor. These processes help the batter puff.

The bubbles need something around them. Flour mixed with liquid makes a thick batter that holds gas for a while. Heat then changes that batter: starch takes up water and swells, and proteins change shape and join into a firmer structure. The pancake can keep many of its tiny spaces after it leaves the pan.

A bubble reaching the top can open a small hole. Inside, other bubbles leave pockets surrounded by cooked material. That is why a cut pancake is not a solid slab. Its softness comes partly from all the spaces interrupting the material you bite through.

The hot surface also dries and browns the outside. The inside stays moister and softer because it is farther from the pan and contains more water. A pancake is therefore several changes happening together: a gas-producing reaction, expanding bubbles, a setting structure and a browning surface. Different recipes balance them differently.