10% of your brain

Why do people (which people??) say that humans only use 10% of the brain? This has become a modern myth with no discernable origin. It is completely and utterly untrue. WE USE ALL OF OUR BRAINs and there is nothing scientific to back up the widely pronounced claim. I am a fan of science fiction but not of fiction posing as science!! Corporations have used this claim as a marketing strategy (american airlines and this ad for satellite tv, for example). Its a bit surprising that no one in any of the companies that represented this "information" or any of the individuals that have said the statement flippantly (likely to make a point in an argument) had the thought to find a source or any reasonable proof. Why do we as humans like to consider ourselves stunted, physically unable to fulfill our potential?
I have a few guesses at the distant origins of this statement, perhaps it is a made up statistic originally used to demonstrate the difference between how much of the brain lights up when consciously attending to something when not consciously attending. The running, accepted theory is it is a misinterpretation of Pierre Flourens, the french physiologist who was the first to notice that lobotomies didnt affect all the functions of bird and rat brains in the 1800s. This led to the understanding of localised brain activity from structures in the brain evolved for specialised activity.Why would our brains evolve to this size if, it isn't all used?
George Orwell said, "Myths which are believed in tend to become true..." and if our brains at their current size of about 3lbs had 90% of it removed.. we would have a brain that is equivalent in size and weight to a sheep's brain. Don't be a sheep, keep using 100% just like you already do.