Do we use 100% of our cerebral capacity?
The idea that we only use 10% of our brain capacity is a myth.  There is a great article at wired.com that discusses the myth and it's history.  

There is really no reason to evolve an entire brain of which only 10% is used.  One great point that they make is that minor brain damage can cause devastating effects, not what you would expect if you had 90% spare capacity waiting around.

The entire brain is rarely firing all at once, and it is hard to average how much is being used because it changes with activity.