Why does bread rise?

Bread rises because yeast feeds on sugars in the dough and gives off carbon dioxide. The dough's gluten makes a stretchy web, so the gas stays caught in bubbles instead of drifting away. As those bubbles expand, the whole lump of dough grows taller and softer.

When bread goes into the oven, the yeast stops living, but the bubble spaces remain. The loaf firms around them, which is why a slice has tunnels, holes, and springy crumb instead of being a flat, heavy cake of flour.