Exercise can sharpen the mind by repairing the brain's protective shield. Researchers have found that physical activity prompts the liver to release the enzyme. This enzyme removes a harmful protein that causes the blood-brain barrier to become leaky with age. In older mice, reducing this protein reduced inflammation. At the same time, reducing the protein improved memory. The discovery points to a pathway from the body to the brain. This mechanism could inspire new therapies for Alzheimer's disease.