“The Babel fish,” said The Hitchhiker’s Guide to the Galaxy quietly, “is small, yellow, and leechlike, and probably the oddest thing in the universe. It feeds on brainwave energy received not from its own carrier but from those around it. It absorbs all unconscious mental frequencies from this brainwave energy to nourish itself with. It then excretes into the mind of its carrier a telepathic matrix formed by combining the conscious thought frequences with nerve signals picked up from the speech centers of the brain which has supplied them.”
1979/2005 p. 58-59