Why bread rises

Bread dough starts as a small, heavy lump. In a warm spot, it slowly grows much bigger. What makes it grow?

It grows because of yeast, a tiny living fungus. The yeast feeds on sugar from the flour and gives off a gas called carbon dioxide.

The gas forms tiny bubbles all through the dough. The longer the yeast feeds, the more gas it makes.

When flour gets wet, some of its proteins form stretchy strands called gluten. Kneading pulls them into a strong, stretchy net.

This net traps the bubbles like small balloons. As they fill with gas, they stretch, and the dough rises.

In the oven, the heat makes the gas expand, so the bread puffs up a bit more. Then the dough firms up around the bubbles and becomes bread.

The holes in a slice are where those bubbles were.

Yeast makes the gas, gluten traps it, and the oven sets the bread around it.