(I never could figure out where the phrase originated, you know something like "people only use 10% of their brains". Who made the measurements, and how?)

I read somewhere that the story behind this is that some researcher
showed that only 10% of the neurons in our brain are firing at any
given time, and the statistic has been quoted out of context ever
since. If anything close to 100% of someone's neurons fired at
once they would have a massive epileptic fit.