古有照妖镜今有卸妆水
[gŭ yŏu zhào yāo jìng jīn yŏu xiè zhuāng shuĭ]
In ancient times there was a mirror that revealed demons; today we have makeup remover. It refers humorously to the concept that in ancient legends, a special mirror can see the true form of monsters and spirits; now instead, makeup removers reveal the true face of people by washing off their makeup. A metaphor highlighting how superficial appearances hide true selves.