Six years ago, on an early morning in September, Molly Birnbaum was out for her regular jog when she was hit by a car. Her pelvis was shattered, her skull fractured, her knee torn. Yet for her, the most serious damage was far less visible: she lost her sense of smell. Birnbaum, now 29, was an aspiring chef, and the loss meant the end of her career. It also meant something else, something that was potentially even more life-changing. “I felt like I lost a dimension of my memory,” she says. “It made me worried about the future. If I couldn’t smell ever again, was I losing this important layer?”