The last words that Liviana Sulzer spoke, 18 months ago, were very much in character: "Now it's time for a song." This was often how she felt, living as she did inside a toddler movie-musical, where even just a spilled cup of milk could get her up onto a chair, twirling with her arms out wide and singing as loud as she could manage: We just spilled our milk … It was messy on the table, and then we cleaned it up … And noooow it's aaaaall cleeeaaaned up! When the song was over, she'd bend toward her brothers, ages 6 and 1, in a deep and gracious bow. It was May 2020--a week before Livie's fourth birthday--and the kids were playing in the yard. Throughout the Sulzers' quiet neighborhood in Austin, Texas, the Persian silk trees had begun to bloom in pink-tipped puffs. There were flowers in their backyard, too. Livie had a favorite one, purple and about as tall as she was. Iris and, trapped at home by the COVID-19 shutdown, she'd made a game of scooting over to it in her push-car and spilling all her problems. But the loneliest phase of the pandemic, with its makeshift games and spotty child care, was nearly over. Things were getting back to normal. A nanny had started just over a week before, and Livie's mother, Lindsay--a bioengineer and expert in regenerative medicine--was headed to the office for her first day back at work, at a local cell-therapy start-up. Livie's father, James, an assistant professor at the University of Texas at Austin who specializes in rehabilitation robotics, was grading papers in the walk-in closet that he'd turned into a home office. He'd asked his graduate students to propose studies or devices that might one day help a patient recover from a nervous-system injury. The sky was clear and calm and sunny. Livie stood near the center of the yard, 30 feet below the overhanging branches of a pecan tree. Her two brothers were nearby.