Surely you've heard someone say that humans only use 10% of our brains (and some people even less), but that turns out to be a just myth:
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 rumor that it was all the brain was overall and most of the rest was not understood or used as far as we know. 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.
Read more human body myths at Environmental Graffiti: Link