Answer:
In baking, yeast is used as a leavener in bread to make the bread fluffy and light. When mixed with flour and water, the yeast is activated by the water and begins to metabolize the sugars found in the flour (flour is ground wheat, wheat contains sugar). As the yeast metabolizes the flour sugar, CO2 is released which creates air pockets in the bread. These pockets will expand when the dough is placed in the oven and will be trapped as the dough bakes, creating fluffy bread.
