Common sense works surprisingly well in some cases: even as a child I didn't believe the "10% of your brain" thing... think about it: the only way they could know this is if someone had 90% of their brain removed and wasn't affected... and that doesn't seem nearly as likely as people wanting to believe that everyone has vast untapped potential.

And let's not even get started on how/why evolution would provide us with 10x the brainpower we need... is there any precedent for that in evolution? Can cheetas run 10x faster than they normally do just by trying a little harder? Can seals hold their breath 10x longer than normal?