To answer this question, we must first understand how black tea is made. All types of tea come from the same plant, Camellia sinensis, but they are processed differently. Black tea is fully oxidized before being dried and packaged for sale; this process releases polyphenols that give it its deep color and full-bodied flavor. Since polyphenol oxidation also increases levels of caffeine in the leaves, one might assume that all black tea would have similar amounts of caffeine per serving. But there are many factors that affect the amount of caffeine found in any given cup: growing conditions such as soil type or rainfall; temperature during harvesting; processing techniques like fermentation time or drying method; even where on the bush the leaves were picked (younger leaves tend to contain higher levels). The quality of ingredients used by a particular manufacturer may also influence caffeine levels between brands. So while two cups of black tea will likely contain some form of stimulant, knowing which company produced it can help you determine just how much. If a tea drinker wants to get the most energy out of their cup, they should think about all of these things.