Hair pulling

Jack's mother ran into the bedroom when she heard him scream and found his two-year old sister pulling his hair.

She gently released the little girl's grip and said comfortingly to Jack "There, there. She didn't mean it. She doesn't know that hurts".

She was barely out of the room when the little girl screamed.

Rushing back in, she asked "What happened?"

Spoiler:

"She knows now" Jack replied.