Ok, I will accept that you do actually use all of your brain. Perhaps some people do not use all of the brain very well though.

For example, one may use all of his/her brain to further their knowledge and understanding of the way the universe works. However another may use all of their brain to absorb what ever tripe is being dished up on today's episode of Springer.

And perhaps there are some "world leaders" out there that should dedicate at least some of their brain to absorbing the dictionary.

I still chuckle to myself when ever I think of "misunderestimated".