Why does bread rise?

Flat dough goes into the oven and comes out as tall, soft bread. The helper is a tiny living thing called yeast.

Yeast is a fungus, related to mushrooms, and each grain of dried yeast holds millions of cells. When you mix them with warm water and flour, they wake up and start to eat the sugars in the flour.

As yeast eats, it breathes out a gas — carbon dioxide — the same gas that makes soda fizz. The gas forms thousands of tiny bubbles inside the dough. The stretchy dough traps them, and the loaf slowly swells.

In the oven, the heat makes the bubbles expand even more, then sets the dough around them. Cut a slice and you can see the holes where every bubble was. Appam and dosa rise the same way — the batter ferments overnight.