The 10% myth is a widely heard of myth that we only use about 10% of our brain. This makes sense to us because what else explains all of our short comings? Why can't all of us recite pi to a couple thousand places? According to Robynne Boyd in his article "Do People Really Only Use 10 Percent Of Their Brains?" (http://www.scientificamerican.com/article.cfm?id=people-only-use-10-percent-of-brain) this myth can best be traced back to the well-known psychologist William James. James claimed that, "We are making use of only a small part of our possible mental and physical resources," in his work The Energies Of Men.
While this may be true evidence shows that we use all of our brains over about a 24 hour time period (John Henley, a neurologist at the Mayo Clinic in Rochester, MN). While we're not using all of our brain at the same time, each part in our brain has to do with a specific function.
However, every once in a while television gets it right like in the show Kyle XY. In the pilot episode of Kyle XY Kyle, the main character who shows signs of being a sort of super genius, gets taken in for an MRI scan. The scanner shows about 70% of Kyle's brain flaring in electric activity and the doctors think that the machine must be in need of calibration. They explain that an average person will use about 7% of his or brain at any given time and that if someone were to use 70% they would be prone to a multitude of risks such as seizures, strokes, and tumors.
Ultimately, we cannot blame our mental short comings on only being able to use a small portion of our brain at any given time. We have to remember that this is a defense mechanism that keeps us safe from medical injuries.