During one of his famous staining experiments of the late 1800s—the kind that would eventually lead to a cure for syphilis and a Nobel Prize for Medicine—Paul Ehrlich stumbled on a conundrum that would haunt medicine down to the present day. When he injected dye into the bloodstream of mice, it penetrated every organ except the brain. Kidneys, livers and hearts turned a dark purplish-blue, clear and stark under his microscope, but the brain remained a pale whitish-yellow. When a student of his injected that same dye directly into the brain, the opposite happened: the brain itself turned blue, whereas the other organs did not. Clearly, the student thought, a barrier—in German, Blut-Hirn-Schranke—must exist between brain and blood.