We only use 10% of our brain.
William James, a psychologist in the 1800s, once metaphorically used the idea of 10% of the brain being all that was used at one time. This grew into the rumour that we only use 10% of the brain overall and most of the rest was not understood or used as far as we knew.
Actually, the inactive neurons are just as important at any given moment as the ones actively firing at a point in time, and the 10% comes from varying areas at different times. We do in fact use all of our brain.