" The guy said, “If you’d taken that punch on the upper arm, you’d expect one hell of a bruise. Which is exactly what you got. Not on the outside. Not enough flesh. The bruise is on the inside. On your brain. With a twin across the hall, because your brain bounced from side to side in your skull like a goldfish in a test tube. What we call coup and contre-coup. "

― Lee Child , Make Me (Jack Reacher, #20)