The blood-brain barrier (BBB) is one of the body's most selective gatekeepers, a dense wall of blood vessel cells, astrocytes, and pericytes that keeps most molecules, pathogens, and particles out of the brain. That's great news for brain health, but a big problem for trying to deliver drugs, and a growing concern for environmental scientists who report finding nanoplastics inside human brain tissue. Somehow, certain nanoparticles are getting through. The question is: which ones, and how?