“We only use 10% of our brains.”
Ruling:
False. Misunderstood metaphor.
Analysis:
Our brains consist of cells which conduct nerve impulses through electrical and chemical signals, these are called neurons. Neurons do not fire at all times; however, that does not mean that inactive neurons are useless, nor that 90% of the 86 billion neurons that make up the human nervous system are permanently inactive.
See other: Mythconceptions?
Knowledge Guild 
Except for some people I know – you KNOW who you are –!