quote from ... The Guardian Unlimited special report : Debunking some popular medical myths

Often it is said, for example, that we use just 10% of our brains. Magician Uri Geller readily spreads this myth as an explanation for why he can bend spoons; he claims to use more of his brain than the rest of us. Truth be told, we use 100% of our brains - even while watching a silly Uri Geller magic show. That 10% figure was invented in the 30s by ad men in America selling self-help pamphlets. "Scientists say you only use one-tenth of your brain," the ads said. "Wake up to your true potential."

In the 19th century, scientists did indeed determine that certain parts of the brain didn't seem to have any obvious function (such as moving a limb) when stimulated by an electrode. They called these regions "the silent cortex" and later learned that these regions were responsible for the very traits that make us human: language and abstract thought.

How can we be sure that Geller is not even 10% right about the brain? For one, commonsense: never has a doctor said, "You'll be fine.The bullet is lodged in the 90% part of the brain you don't use." Biologically, any part of the body will deteriorate without use. Legs shrivel in a cast, and neurons in the brain die as a result of diseases such as Alzheimer's and dementia. And if you want proof in pictures, modern scans all show that the entire brain is active.