Years ago, in a Brooklyn high school, a door slammed. Christopher Emdin, then a 10th-grader, immediately ducked under his desk. His math teacher accused him of being a clown and sent him to the principal's office.
Emdin wasn't being a clown.
A couple of days before, there had been a shooting just outside his apartment building. He thought the slamming door was a gun shot. His jump for cover was instinctual.