EACH NIGHT AT DINNERTIME, a familiar ritual played out in Michael Green's home: He'd slide a stainless steel sippy cup across the table to his two-year-old daughter, Juliette, and she'd howl for the pink plastic one. Often, Green gave in. But he had a nagging feeling. As an environmental-health advocate, he had fought to rid sippy cups and baby bottles of the common plastic additive bisphenol A (BPA), which mimics the hormone estrogen and has been linked to a long list of serious health problems. Juliette's sippy cup was made from a new generation of BPA-free plastics, but Green, who runs the Oakland, California-based Center for Environmental Health, had come across research suggesting some of these contained synthetic estrogens, too.