Does a writer use only 10% of his keyboard? A billiards player only 10% of his cue? I could go on and on, but the answer is yes! The billiards player strives to strike the cue ball perfectly with only the tip of his cue, which is only a small fraction of its entire size. The fact is, achieving optimal results means utilizing and focusing energies on the bit that really matters.
Take the keyboard for example. As we type we are only hitting one key at a time, some faster than others, but only one key nonetheless. If I want to write like Shakespeare, would it help by hitting all keys at once? I'm sure you'll agree using 100% of the keyboard will produce gibberish.
Ditto to Mr. Brain - we focus our attention only to the task at hand, devoting energy and resources that are applicable. The rest of the brain is the supporting cast, present and ready to be called upon. Most of them are handling other unique tasks as well studies show that our eyes move and adjust over 1000 times a second, while our nerves send back millions of minute signals (in that same second)!
Surely all these essential bodily functions are not on auto pilot nor left to chance. And that's where the other 90% of the brain' comes into play. They handle the stuff we can't live without, literally - expanding and collapsing of the lungs, contractions of the heart, maintaining chemical balances in the blood, etc. So the next time somebody drops the '10% of the brain' fact on you, your reply should go something like "..and boy am I thankful for that!".
But don't be fooled, despite being compared to a keyboard and a cue stick, the brain is an intricate mesh of Neurotransmitter systems loosely comprised of the Noradrenaline system, Dopamine system, Serotonin system, and Cholinergic system. These are responsible for all our emotions, and never take a break during the course of our lives. But enough of the technicalities.
Like most other internal organs, the brain can serve as nourishment. For example, in the Southern United States canned pork brain in gravy can be purchased for consumption as food. This form of brain is often fried with scrambled eggs to produce the famous "Eggs n' Brains".
Do be warned, however, that consuming the brain and other nerve tissue of animals is not without risks - Brain consumption can result in contracting fatal transmissible spongiform encephalopathies such as Variant Creutzfeldt-Jakob disease and other prion diseases in humans and mad cow disease in cattle. (Sounds like a bad-ass form of Karma to me.)