I often hear it said that we only use about 10% of our brains with the implication, explicit or implicit, that we might be able to find ways to use more of our brain capacity for thinking. Nothing could be further from the truth. The brain is chock-full with needed function, with no room for waste.
The brain has to construct our entire sensory experience, facilitate coordinated motor activity(allow for movement), store all of our relevant memories, calculate the costs and benefits of behavior, facilitate mood and emotional responses, provide a networked inborn language development faculty, and provide for many reflexes, among other duties.
It wouldn't make sense to have unused capacity in the brain, as the resources needed to keep neurons(brain cells) alive, along with carrying the dead weight in the head would be energetically wasteful. This is why unused brain cells naturally die in a process known as apoptosis.
This is just one of many myths about the brain.