The mother ran into the bedroom when she heard her seven-year-old son scream. She found his two-year-old sister pulling his hair. She gently released the little girl's grip and said comfortingly to the boy, "There, there, she didn't mean it. She doesn't know that it hurts." He nodded his acknowledgment, and she left the room. As she started down the hall the little girl screamed. Rushing back in, she asked, "What happened? The little boy replied, "She knows now."