The adult human brain was once understood as being incapable of forming new neurons. The common understanding was, in fact, that the brain began losing neurons. The actual evidence and our understanding now, is that adult brains can form new neurons. Additional evidence has been discovered that establishes exercise has positive effects on human brains, notable as brains age. Exercise may also reduce the risk of Alzheimer’s disease and other neurodegenerative conditions. This evidence prompts a key question: Why does exercise affect the brain?