Do we really only use 10% of our brains?

I've always hated the myth that we only use 10% of our brains. This is usually a myth that is spread to try & get you to buy some kind of speed reading course when I'd much rather spend my money on a Hilton Head rental or stamps for love letters to write to Adam Brody.

I'd argue that we always use 100% of our brains, just on somedays our 100% doesn't feel as good as at other times because we are tired or distracted. According to this article that I'm too selfish to share right now brain imaging shows that we use all of our brains. The brain has so many different duties that we can't say that any part doesn't function because we don't know what everything does.