Leaves of three, let it be! Why does poison ivy produce such a terrible, itchy rash? Young outdoor enthusiasts get acquainted with urushiol, poison ivy's rash-producing natural oil, with a video from the American Chemical Society's Reactions playlist. The narrator enlists the help of a chemist to explain how urushiol works in the body, its freakishly long lifespan outside of the poison ivy plant, and how to get some relief from the resulting rash.