“Though we know precious little about how the brain works, our evolutionary history tells us this: The brain appears to be designed to (1) solve problems (2) related to surviving (3) in an unstable outdoor environment, and (4) to do so in nearly constant motion. I call this the brain’s performance envelope. Each subject in this book—exercise, sleep, stress, wiring, attention, memory, sensory integration, vision, music, gender, and exploration—relates to this performance envelope. We were in motion, getting lots of exercise. Environmental instability led to the extremely flexible way our brains are wired, allowing us to solve problems through exploration. To survive in the great outdoors, we needed to learn from our mistakes. That meant paying attention to certain things at the expense of others, and it meant creating memories in a particular way. Though we have been stuffing them into classrooms and cubicles for decades, our brains actually were built to survive in jungles and grasslands. We have not outgrown this.”