Sometimes you might see a claim like “you only use 10% of your mental potential/brain capacity/brain power/etc.”, and usually that sentence is followed by description like “…and now we show how to tap into the other 90%”, followed with an imaginative, quick, effortless way how you “tap into” that untamed reservoir of turbo-charged neurons.

So what’s wrong with that?

The 10% brainpower claim is a myth – there is no evidence supporting such a claim. Snopes has a good article about it, it’s worth a read.