The Real Reason for White Hair?

One day, a little boy is sitting and watching her mother do the dishes at the kitchen sink. He suddenly notices that his mother has several strands of white hair sticking out in contrast on her brunette head. He looks at his mother and inquisitively asks, "Why are some of your hairs white, Mum?" His mother replied,"Well, every time that you do something wrong and make me cry or unhappy, one of my hairs turns white." The little boy thought about this revelation for a while, and then said, "So, Mum, how come ALL of grandma's hairs are white?"