Before dough is placed in the oven, bakers slash the top to allow room for oven-spring. The heat of a 500 degree oven causes the gases of water vapor and carbon dioxide to superheat. The gases expand rapidly, but the fine netting of gluten built by kneading the dough contains the gases, preventing most of them from escaping to the atmosphere. Some gases, including volatile organic compounds, do escape and the smell of baking bread comes from the oven in short order.
But the gases trapped by gluten strain at their walls and the bread rises while it is baking. A well-trained baker will get the timing and placement just right so that an ear forms. A raised wing of baked bread, crispy and inviting, calls out to eaters.