For half a century, neuroscientists thought the human brain contained 100 billion nerve cells. But when neuroscientist Suzana Herculano-Houzel devised a new way to count brain cells, she came up with a different number — 86 billion. Herculano-Houzel, now an associate professor of psychological science at Vanderbilt University, describes her research and explains how we "lost" 14 billion neurons overnight.