That we only use 10% of our brains is a myth. In reality, our brain "usage" is far more complex than this. While we do have cognitive load limitations (e.g. most of us aren't very good at remembering more than about seven discrete pieces of information at a time), our brains don't work exactly as a computer does. The processing that our brain undertakes is mediated by very complex networks of brain regions which perform specific functions (e.g. recognizing an array of shapes in the visual field as a face, the fusiform face area), the connections between those regions, and regions whose functions we do not yet understand.