A tablespoon of it will kill you, but most of us feel like death without it: We’re talking about caffeine. Inspired by a listener question—does green tea have more or less caffeine than black? And what about yerba mate?—Cynthia and Nicky spend this episode exploring the history and science of the world’s most popular drug. Listen in as we discover the curious effect of birth-control pills on how our bodies process it, calculate how much of an edge it gives athletes, and learn what dolphin dissection and the American Constitution have to do with each other, and with caffeine.

Caffeine is a miracle of plant chemistry—one that evolved on four separate continents, though experts are not entirely sure why. The prevailing hypothesis has been that caffeine functions as a pesticide, but, in this episode, food-science guru Harold McGee shares more recent science that seems to contradict that. In any case, humans quickly figured out that caffeine-rich plant products—cacao beans, coffee berries, tea leaves, kola nuts, and more—made them feel great: sharper, less tired, and even a little stronger. Murray Carpenter, author of Caffeinated: How Our Daily Habit Helps, Hurts, and Hooks Us, gives us the scoop on the science behind how caffeine affects our brains and bodies, while author Bennett Alan Weinberg demonstrates caffeine's impact by telling us the fascinating story of what happened when the stimulant finally arrived in Europe, a continent without a native source of its own. And, of course, we answer our listener Erik's question, and not just by saying, “It’s complicated”—although, of course, it is. All sorts of variables, from particle size to roast darkness to steeping time, affect how much caffeine is in your afternoon pick-me-up.