One day a little girl was sitting

One day a little girl was sitting and watching her mother do the dishes at the kitchen sink. She suddenly noticed that her mother has several strands of white hair sticking out in contrast on her brunette head. 

She looked at her mother and inquisitively asked,

"Why are some of your hairs white, Mom?" 

Her mother replied, "Well, every time that you do something wrong and make me cry or unhappy, one of my hairs turns white." 

The little girl thought about this revelation for a while and then said, "Mom, how come ALL of grandma's hairs are white?"