As Whiteman explains in this episode of the UC Berkeley video series 101 in 101, that’s partially because some plants, like coffee, have developed caffeine at least in part as a poisonous defense against chewing insects and other enemies. As it turns out, what harms insects and their tiny bodies gives many of us larger humans a brisk chemical jolt to start the day. Many plant species make poisons that are lethal to insects, but some insects in turn have coevolved with these plants to even take advantage of the poisons.