Ten Percent of Your Brain
In the 1960s, Dr. Carl Mellon performed a series of intricate experiments into the workings of the human mind. He discovered that even though humanity was in possession of the most complex brains on the planet next to those of dolphins, we are "vastly underperforming vis-á-vis our potential" because we only use 10% of our brains.
It is strongly believed that harnessing a significant portion of that unused 90% would allow humans to tap into latent psychic powers. As psychic idol Uri Geller says, "If you are using more than 10% of your mind, you will buy my book!"
Behind the Legend
Dr. Carl Mellon's research into the use of the brain was revolutionary for its day. He did indeed discover that humans use only (approximately) 10% of their brains. However, at the time he published his results, nine out of ten of his colleagues disagreed with his conclusions.
Fortunately for Dr. Mellon's legacy, time has vindicated him. The 10% figure has been verified with a margin of error of 1% using a series of intricate PET scans (high-resolution radiation scans of the brains of dogs and cats). Unfortunately for Uri Geller and his ilk, scientists also found out what the other 90% was capable of.
It turns out that the unused 90% of the brain is largely devoted to functions that may have been handy at one point in our evolutionary history but are now out of date. For example, an additional 10% of the brain is devoted to such tasks as breathing through gills, identifying each of 37 subspecies of saber-tooth tiger, believing in fairies and their like, and maintaining fealty to a monarch. In men, the remaining 80% is untapped potential for thinking about sex. What the unused 80% of women's brains are capable of is unknown, because the men who discovered the masculine 80%'s purpose were so excited about their results that they went out to tell their buddies and forgot all about finishing their research.