Well, you know, humans only use 10% of their brains, so if scientists could tell us how to use the other 90%, there would be no more
It's not true.
The brain is an amazing biological/electrical/chemical structure of up to an estimated 100 billion neurons. We can't survive without it. While it's possible for some people to recover from brain damage and possible for some people to function with certain parts of their brains missing, the idea that we are not using 90% of our brains is, well, mind-boggling. Think of the things your brain automatically does for you: breathing, circulating blood, digesting food, sensing stimuli, regulating body temperature, etc. On top of that, add in all the things that you consciously ask your brain to do for you: walking, talking, reading, thinking, moving, etc. Doesn't sound like a 10% proposition to me. How about you?
Here's a brief article courtesy of the Association for Psychological Science (APS) addressing this myth with an excerpt from the book 50 Great Myths of Popular Psychology.
Another take on the ten percent myth can be found at Snopes.com, a great resource for sorting out fact from urban legend.
