 Brain cells produce waste. So Jonathan Kipnis of Washington University in St. Louis says the brain has a system to clean itself. You have the water pipes and the sewage pipes. And so water comes in clean and then you wash your hands and then the dirty water goes out. But Kipnis says scientists haven't known what powers this system until now. Two independent mouse studies found that a distinct pattern of brain waves that appear during sleep turned neurons into tiny pumps that push fluid out of the brain.