Severus went very still. The last time he had been struck like that, he had been very young. It had brought back an instant flood of horrible memories, of his drunken father and his hysterical mother, both of them festering wounds of people, taking out their illnesses on their misfortunate, regrettable child. A slap across the face was hardly the worst he had suffered at the hands of others, but he would not forget the instant fear he had felt in that moment, fear he had tried so hard to suppress. The fear of a chastised, vulnerable child, unsure of exactly why they were being harmed by someone they loved.