Next time you sip your tea or bite into a bar of chocolate, or load up your grocery cart with other treats, spare a thought for the underpaid or unpaid workers who made it possible. Modern slavery comes in many guises, and politics professor Genevieve LeBaron of the University of Sheffield in England, who's done field studies on the subject, is here to tell you how it happens, and what you might want to look out for as you shop.