Perhaps you've seen Morgan Freeman saying it most recently in the trailer for Lucy.
It's not true.
We use 100% of our brains. So can way cut that bullshit out already?
"Though an alluring idea, the '10 percent myth' is so wrong it is almost laughable, says neurologist Barry Gordon at Johns Hopkins School of Medicine in Baltimore. ... 'It turns out though, that we use virtually every part of the brain, and that [most of] the brain is active almost all the time,' Gordon adds. 'Let's put it this way: the brain represents three percent of the body's weight and uses 20 percent of the body's energy.'"