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Some five ounces of clear fluid fills the spaces between your brain and your skull. This brain juice, or cerebrospinal fluid, cushions against injury, supplies nutrients and clears away waste. Your body can make as much as a pint of fresh stuff every day to replace the old.

But for 150 years, scientists have puzzled over how the used cerebrospinal fluid leaves the brain to make room for more. New research, published Wednesday in Nature, has fin