Of all the things humans rely on plants for—sustenance, beauty, medicine, fragrance, flavor, fiber—surely the most curious is our use of them to change consciousness: to stimulate or calm, fiddle with or completely alter, the qualities of our mental experience. Take coffee and tea: People around the world rely on caffeine to sharpen their minds. But we do not usually think of caffeine as a drug, or our daily use as an addiction, because it is legal and socially acceptable. So, then, what is a “drug”? And why, for example, is making tea from the leaves of a tea plant acceptable, but making tea from a seed head of an opium poppy a federal crime?