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.