Maybe we do use only 10% of our brains:

Magnetic Resonance Imaging (MRI) scans of a 44-year-old man's brain show a huge fluid-filled chamber called a ventricle taking up most of the room in his skull, leaving little more than a thin sheet of actual brain tissue, in this handout image released by French researchers July 19, 2007. The man with the unusually tiny brain has managed to live an entirely normal life as a married civil servant with two children despite his condition, according to the researchers.

(Thanks, Steve!)