Ten-percent-of-the-brain myth
The ten-percent-of-the-brain myth or ninety-percent-of-the-brain myth states that humans generally use only one-tenth (or some other small fraction) of their brains. It has been misattributed to many famous scientists and historical figures, notably Albert Einstein.
Source: Wikipedia — Ten-percent-of-the-brain myth (CC BY-SA 4.0)