Of all the things humans rely on plants for–sustenance, beauty, fragrance, flavor, fiber–surely the most curious is our use of them is 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. We don’t 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?