Do humans use only 10% of their brains?

Over a normal day, fMRI and PET light up virtually every region. There's no idle 90% — damage almost anywhere produces a measurable deficit, which wouldn't be true if most of the brain were spare capacity.

Steelmanning the kernel of truth: at any single instant only a small fraction of neurons fire, and glia outnumber neurons. But 'we only use 10%' as a claim about untapped *capacity* doesn't follow from that — and that's exactly how it's always sold.