No, not even close. This pervasive pop-culture myth—one survey found that 50 percent of science teachers believed it was true—has no basis in reality. We use 100 percent of our brains every day, as is clearly shown by functional magnetic resonance imaging scans. Neurons only make up 10 percent of the cells in our brains but the other 90 percent work full-time, maintaining homeostasis, providing structural support, and removing pathogens. The source of the famous notion is pioneering psychologist William James, who once write that “we are making use of only a small part of our possible mental and physical resources,” and he was right—but our untapped potential has little to do with our brain cells.