When tea oxidizes, it undergoes a series of chemical reactions that can cause changes in its flavor, aroma, and appearance. Oxidation is a natural process that occurs when tea leaves are exposed to air. During oxidation, enzymes in the tea leaves react with oxygen in the air, which can cause the leaves to darken and the flavor and aroma of the tea to change. This process is similar to the browning of fruit when it's exposed to air. The extent of oxidation depends on the type of tea and how it was processed. For example, black tea is fully oxidized, while green tea is not oxidized at all. Oolong tea falls somewhere in between, as it's partially oxidized. During oxidation, the flavor and aroma of tea can change from fresh and grassy to more complex and rich. For example, in black tea, oxidation causes the tea to develop a malty, earthy flavor, while green tea has a more vegetal and fresh flavor. The appearance of the tea leaves can also change during oxidation. For example, black tea leaves are fully oxidized and turn a dark brown or black color, while green tea leaves are not oxidized and retain their green color. In summary, oxidation is an important factor in determining the flavor, aroma, and appearance of the tea. It's a natural process that can enhance the flavor of some teas, while detracting from others, depending on the type of tea and how it's processed.