Jess hits his little sister May Belle in the face, hard.

Jess hits May Belle after returning from "paying (his) respects" to the family of his best friend Leslie Burke.  Jess has been stunned by her death, and completely unprepared to deal with his tumultuous feelings of guilt, bewilderment, and grief.  He at first refuses to believe that Leslie is gone, literally trying to run away from the fact, "knowing somehow that running (is) the only thing that (can) keep Leslie from being dead".  After his father follows him and tenderly brings him back home, Jess sleeps a long while and awakens confused, and it is only during his visit to Leslie's family that the reality of the young girl's irrevocable absence really hits him.  Jess then reacts with rage, at the world, at the people who are mourning, and at Leslie herself.  He realizes, "Leslie (is) gone...she had failed him...went and left him...made him leave his old self behind and come into her world...left him stranded...like an astronaut wandering about on the moon...alone".

Jess runs home from the Burke's house and as he "bang(s) through the door" of his own home, May Belle is standing there, asking with wide-eyed, excited childish innocence and obliviousness, "Did you see her? Did you see her laid out?"  It is at this point that Jess hits May Belle, "in the face...as hard as he had ever hit anything in his life" (Chapter 12).