Brother and Sister explained simply

A certain man had two children, a boy and a girl: and the boy was as good-looking as the girl was plain. One day, as they were playing together in their mother's chamber, they chanced upon a mirror and saw their own features for the first time. The boy saw what a handsome fellow he was, and began to boast to his Sister about his good looks: she, on her part, was ready to cry with vexation when she was aware of her plainness, and took his remarks as an insult to herself. Running to her father, she told him of her Brother's conceit, and accused him of meddling with his mother's things. He laughed and kissed them both, and said, "My children, learn from now onwards to make a good use of the glass. You, my boy, strive to be as good as it shows you to be handsome; and you, my girl, resolve to make up for the plainness of your features by the sweetness of your disposition."