If you've ever been on the freeway and saw the guy next to you holding his coffee with one hand, texting with the other, and steering with his kneecap while doing 80, you might find it quite plausible that humans only use 10% of their brain.
This is actually a scientific urban legend, though, and quite far from the truth. The man you see is engaging many parts of his brain - the driving uses the cerebellum, the texting uses his frontal lobe, reading his texts uses his visual cortex. He finally heard you honking after his kneecap steered into your lane. That's the temporal lobe. Although this man may not be using his brain very well, he is still using it.The myth that we only use 10% of the brain is roughly 100 years old, and is somewhat ingrained in our culture. functional MRI however shows that even with simple tasks, such as watching a movie, large areas of our brain show increased blood oxygenation.
Even parts of the brain which can't be used for their designed purpose get repurposed. For example, blind people can locate sounds using their visual cortex - demonstrating that in the brain, real-estate is so valuable that vacancies are rare. Given that damaging nearly any area of the brain leads to some observable deficit, all areas of the brain are not only used, but they are used quite importantly.
Even if you wanted to shut off 90% of your brain, you would be unable to, since so many functions ranging from breathing to vision all operate there. There is even a brain network that actually activates when we are at rest- called the default mode network, which some hypothesize is related to consciousness and self-referential thought.
There may be an additional evolutionary reason for our brains to be perpetually "active." Our ancestors hardly had the chance to sleep in an air-conditioned house with a deadbolted door, a security system, and a yappy dog. There was danger all around, and shutting off the brain during periods of sleep or even daydreaming could make them susceptible to real threats.
Because of this, our brains might have developed to be in "standbye" mode rather than being shut-off, which means that even now people do use most of their brain, even when they act like they don't have one.
Front page image credit: "Lucy" by Besson and Europacorp