This term explains the most common form of preparation for tea leaves, and is correct in most cases. Once a tea leaf has been plucked, it begins to wither and become oxidized. Certain enzymes in the cells of the leaves react to the oxygen in the air. This breaks down chlorophyll in the leaves, causing them to darken and changing their shape and flavor. Heating the leaves can interrupt the process, allowing tea growers to control how much oxidation the leaves experience. The amount of oxidation is one of the primary factors that determines what type of tea is made with the leaves.