With my diligence, carefully
I examined the body
And saw this as it really is —
Both within and without.
Because Nanda was so taken up with her physical beauty, the Buddha used his psychic powers to conjure up the vision of an even more beautiful woman, who then aged visibly and relentlessly before her very eyes. Thereby Nanda could see, compressed within a few moments, what otherwise one can only notice in people through decades — and often because of proximity and habit one does not even fully comprehend: the fading away of youth and beauty, the decay, the appearance of wrinkles and gray hair. The vision affected Nanda deeply; she was shaken to the center of her being.
After having shown her this graphic picture, the Buddha could explain the law of impermanence to her in such a way that she penetrated the truth of its completely, and thereby attained the knowledge of future liberation — stream-entry. As a meditation subject the Buddha gave her the contemplation of the impermanence and foulness of the body. She persevered for a long time with this practice “faithful and courageous day and night”…
Because Nanda had been so infatuated with her physical appearance, it had been necessary for her to apply the extreme of meditations on bodily unattractiveness as a counter-measure to find equanimity as balance between the two opposites. For beauty and ugliness are just two kinds of impermanence. Nothing can disturb the cool, peaceful heart ever again.
Buddhist Women at the Time of The Buddha
(By Hellmuth Hecker, translated from German by Sister Khema)