And, the answer is: no. No, we don't only use 10% of our gray matter; in fact, we're using most of our brain most of the time. How do we know? First, neural imaging techniques let us see indirectly that the brain is active all the time — even when you're doing routine tasks, or nothing at all. Also, if we didn't use all of our brain, then it wouldn't be such a huge deal when we injure or lose a part of it. But, it's absolutely a huge deal. And, perhaps our least glamorous piece of evidence that our brains are so active is that we have to keep them fed with hundreds of calories every day, just to keep them functioning.
This Is Why We Shouldn't Throw The Word Claustrophobia Around So Casually