In museum drawers and fossil beds, small smooth stones have quietly preserved one of evolution's most consequential secrets. Researchers from Japan and South Korea, analyzing gastrolith wear patterns across dozens of dinosaur and living bird species, have found that the gradual transfer of digestive labor from jaw to stomach was not a footnote to avian evolution — it was its engine. As teeth vanished in certain theropod lineages, the stomach became a mill, shifting the body's center of gravity and making bipedal flight-ready posture not just possible, but inevitable.