In 1981, a chemist at a pharmaceutical company accidentally brushed her hand across her mouth and discovered one of the sweetest substances ever found — a compound she wasn't supposed to be making, in a lab where she wasn't supposed to be eating. But the story of sucralose is really a story about how the human body recognizes sweetness at all, and why scientists spent decades arguing over a receptor they couldn't see, couldn't isolate, and weren't even sure existed.