Why does bread rise?
For grown-ups: the science
Bread rises because yeast makes tiny bubbles of carbon dioxide gas inside the dough.
Yeast is a single-celled fungus that ferments available sugars in dough, producing carbon dioxide and ethanol. Gluten and other dough structures trap much of the gas, expanding the dough. Oven heat expands gases and sets the dough's structure as starch gelatinizes and proteins coagulate.
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Transcript
Look at the dough puffing up in its bowl. Why does bread rise?
Bakers mix yeast into dough. Yeast is a tiny living thing that eats some of the dough's sugar.
As yeast eats, it makes carbon dioxide gas. The gas forms little bubbles trapped in the stretchy dough.
Those bubbles grow and push the dough upward. Heat in the oven helps the bread stay tall and fluffy.
So bread rises because yeast makes tiny gas bubbles in the dough.