KRISH SATHIAN: So there is this myth that we only use 10% of our brains. And that's really simply not true. We are probably using all of our brain much of the time and much of our brain all of the time.
Even when you're engaged in a task, and some neurons are engaged in that task, the rest of your brain is occupied doing other things-- which is why, for example, the solution to a problem can emerge after you haven't been thinking about it for a while or after a night's sleep. And that's because your brain is constantly active. If it were true that we only use 10% of the brain, then we could presumably sustain damage to 90% of our brain-- with a stroke, or something, or brain injury, or something like that-- and not have any effects, and that's clearly not true.
So the question of where the 10% myth originated is an interesting one. The answer is, it's murky. According to some accounts it dates back to William James, a professor of psychology at Harvard, who suggested based on his work that humans don't really use their brain to its full capacity. And then, somehow, this got distorted over time and somebody slapped a figure on there and became the 10% myth as we understand it now.