Thanks to a blood-brain barrier—a natural wall that protects the brain tissue—the organ never touches blood, thus protecting it from microbes, viruses, and other pathogens.

To get nutrients to brain tissue and remove its waste, the brain makes a liquid called cerebrospinal fluid. But exactly how the fluid removes gunk generated by brain cells wasn't certain until now.

Experiments in the 1950s and '60s hinted that diffusion—the passive method by which, say, food coloring spreads out in a glass of water—moved cerebrospinal fluid around the brain.

Yet this process is too slow to explain the brain's lightning-fast activity and immaculate cleanliness.