As you said, we don’t use only 10% of our brain. It would be like saying that we use only the 10% of our stomach. We use all of our brain, but not at the same time. The brain is made of numerous parts that do different things: when I’m laying on the bed thinking about a math problem, I’m using parts of my brain that are different from when I’m running a bike race. Also, the parts of our brain don’t work like there is only ON/OFF. In different kind of activity (like: reading, talk to a friend, give a speech in public) we use often the same part, but at different “levels of activation”.

So, it’s a myth that we use only 10% of our brain. The brain is much more busy.