Answer:
brain
Public Domain Image, source: Christopher S. Baird.
Healthy humans use all of their brain. There is no part of the brain that goes unused. Certain tasks work certain parts of the brain more, but they all play important roles, as explained by neurobiologist Dr. Eric Chudler. Brain maps, as found in modern anatomy books, indicate that each part of the brain has a specific function essential to a healthy human. If there were a part of your brain that really went unused, then you could safely damage that part in an accident with no ill effects. But decades of medical records show that damage to any part of the brain has severe effects. If 90% of the brain were not used, then 90% of the brain tumors would cause no problem. Imagine brain doctors telling 90% of their cancer patients, "I have good news and bad news. Bad news: you have a brain tumor. Good news: it's in the part of the brain that you will never use." The thought is absurd.
If the 10% myth is instead supposed to mean that humans only use 10% of their brain in a given moment, it is still false. The brain is not a collection of independent machines that are turned on or off depending on whether you are reading or singing. Rather, brain functions emerge as a complex interplay of many parts of the brain. Physiologically, nerves are like muscles in that they degenerate when unused. If 90% of the brain went completely unused, then that portion would degenerate significantly. But brain scans of a healthy person reveals all parts to be intact. This myth was propagated by authors trying to sell books on mystical ways to unlock your hidden potential, claiming that unused brain power could be tapped using the methods in their books. The greatest danger to your brain is not the possibility that a large portion is going on unused. Rather, the greatest dangers are stroke, Alzheimer's disease, and tumors. The best ways to protect yourself from such risks include eating healthy, exercising, and getting enough rest. Do you really want to use your brain to its full potential? Then put down your unlocking-hidden-brain-potential book and go on a run.
Explanation:
