The brain acquires certain skills-from visual perception to language-during critical windows, specific times in early life when the brain is actively shaped by environmental input. Scientists like Takao K. Hensch are now discovering pathways in animal models through which these windows might be re-opened in adults, thus re-awakening a brain's youth-like plasticity. Such research has implications for brain injury repair, sensory recovery, and neurodevelopmental disorder treatment. In addition, what we know today about these critical windows of development already has enormous implications for social and educational policy.