Actually thats incorrect. The actual theory surmises that the CONCIOUS mind only uses 10% (or less) of our brain. The rest is used by your unconcious. Ever had an idea pop into your head? ...yeah
Even then the unconcious mind doesn't use your entire brain. Remember, not all the nuerons are firing at once, often only about 10% or even less are firing at any given time. If they were all firing, well, then all those catscans would turn up bright freaking red.