It's been a huge mystery how the brain clears toxins and why clearance slows with age. Researchers showed (in mice) that the brain has a trash chute in the form of microscopic holes right behind the nose. As mice age, those holes narrow and drop in number, cutting total fluid outflow by 40-50%. A single intranasal gene therapy doubled the size of the drainage pipes downstream that allowed fluid flow to be on par with young adult levels within six weeks, even though the original holes remained narrowed.