How Yeast Makes Gas
Yeast is a tiny living organism, a type of fungus. When you mix yeast into bread dough, the yeast cells start eating sugars that are naturally present in flour. As the yeast breaks down these sugars, it produces carbon dioxide gas as a waste product. This is the same gas that makes soda fizzy. The gas gets trapped in the sticky dough, creating thousands of small bubbles.
The Rising Process
As more yeast cells grow and produce more gas, the bubbles expand and push the dough upward. This makes the dough increase in size, or rise. The longer you let the dough sit, the more gas is produced and the more it rises. This process is called fermentation, and it's the same process that happens when making beer and wine.
Temperature Matters
Yeast works faster in warm environments. When dough is warm, yeast cells multiply quickly and produce gas faster, so the dough rises quickly. Cold dough rises much more slowly because yeast is less active in cold temperatures. This is why recipes often tell you to let dough rise in a warm place.
What Happens When You Bake
When you put the risen dough in the oven, the heat kills the yeast and stops the rising process. The heat also causes the gas bubbles to expand even more, making the bread rise one final time in the oven. Then the heat causes the outside of the bread to turn brown and firm, trapping the bubbles inside and creating the light, airy texture of baked bread.
Why Bubbles Matter
The bubbles created by yeast gas are what make bread soft and spongy instead of dense and hard. Without yeast and fermentation, bread would be flat and heavy like a brick. The tiny pockets of air give bread its pleasant texture and make it easier to chew and digest. 
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