Description The 'critical period for brain health' determines who holds the reins of one's own life. In the summer of 2006, strange symptoms began. She smelled bread that wasn't there, yet couldn't smell the roses right in front of her. As a neurologist who knew the correlation between certain causes of dementia and impaired sense of smell, the author began relentlessly tracking her own health from that point on. Finally, in 2015, she received an official diagnosis of Alzheimer's disease. "At last, everything made sense. And the brain scan itself was, as always, beautiful." When we imagine growing old, dementia is one of our greatest fears. Yet scientific evidence is mounting that dementia can be detected before symptoms appear, and that its progression can be slowed. The reason the author remained so composed at the moment of diagnosis was that she had begun changing her lifestyle a full decade earlier. Healthy habits build what's known as 'cognitive resilience.' Even among people who develop dementia, those with greater cognitive resilience experience the onset of cognitive impairment later than those without it. In particular, consistent aerobic exercise can reduce the likelihood of developing dementia by roughly 50 percent. In other words, every choice we make during the 'critical period for brain health' has the power to change the course of our lives. An ordinary kind of courage, spoken from the heart of life's uncertainty Faced with crisis, our brains and bodies forge new connections to resist loss. Life works the same way. When plans fall apart and we're pushed onto unexpected paths, we sometimes find unforeseen gifts waiting there—just as new doors opened for the author in research, lecturing, and writing after she believed her career as a physician had come to an end. "Alzheimer's disease bears witness not only to fear and dread, but also to the hope that exists within us." Rather than surrendering to despair, the author emphasizes focusing on what can be done in this very moment. It's not because she is an exceptionally brave person, but because of the wisdom she gained from walking a path filled with fear, shame, and uncertainty before the rest of us. When we look mortality squarely in the eye, we come to see clearly what truly matters. So much depends on a single life. We all hope to preserve our dignity to the very end, and the closer that hope draws, the more frightening loss and crisis feel. And yet, each of our lives is also a precious legacy, "built together with the people we love." The meaning of life exists somewhere beyond genes and data, beyond fear and probability. The power of taking one step at a time—this is the attitude toward life that the author hopes to share.