Little Johnny's Little Sister

Johnny'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 Johnny, who's just five years old, 

"There, there. She didn't mean it. She doesn't know that it hurts."

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

Rushing back in she said, "What happened?"

"She knows now," little Johnny explained.