How much of the human brain is used?

Answer:
All of it, though not all at once. (If every neuron in your brain was firing uncontrollably all at once, you'd have an epileptic seizure.)
The "10%" figure you'll occasionally see is an old wives' tale/urban legend/gross oversimplification of a misinterpretation of someone who actually had some idea of what they were talking about said.
First answer by Ptorquemada. Last edit by Ptorquemada. Contributor trust: 551 [recommend contributor recommended]. Question popularity: 2 [recommend question].