The myth that we only use 10% of our brains is sticky and gets everywhere, much like glue-dipped belly button fluff. And just like glue-dipped belly button fluff, it’s a nuisance, and can only be combatted with the even-stickier duct tape of truth. Rather than attempting to be exhaustive, I’ll simply appeal to your intuitions to try and make sense of how it’s nonsense.
The urban legend that we only use 10% of our brains is true in the same way it’s true that we only use 10% of a piano. Have you ever seen even a concert pianist press more than ten keys at once? It wouldn’t be too hard – one could probably manage 20 or 30 by adding elbows, and maybe even more with props. But it would sound terrible – all of the informative signal that goes into making music rather than noise is in the choice and timing of the keys that get pressed. It’s the pattern of keypresses that matters.
The same is true of the brain. It may be that only (say) 10% of neurons are firing at a given moment, but that pattern of firing is what matters – the choice and timing of which neurons are active is what constitutes thought.
It is worth noting that, just like the piano, there are bits of your brain that are active more often than others – you can probably get rid of the very lowest and very highest notes without too much of a problem, though occasionally things might sound a little odd. The same is true for people and animals – you can lose a few thousand neurons heading a football or downing shots and no one will notice. But if you lose a big chunk of brain, or multiple keys in a row, then the music is going to sound pretty bad.