Cured leaves are brewed to drink; the various methods of curing produce different teas. Essential oils give the flavor and aroma, tannin gives the "bite" and color. Tea is an astringent and is high in tannin; China tea has more caffeine and is a tonic drunk to clear toxins and regulate the metabolism oolong tea may lower cholesterol. Research on green tea shows it may stimulate the immune system. Cooled black tea bags soothe sunburn.