RECENTLY, the A4 waist (腰) challenge has gone viral (迅速传播) on Sina Weibo. Many Internet users, especially women, have posted pictures of themselves with an A4-sized piece of paper in front of their waistlines. Users win the challenge if their waists are completely hidden behind the 21-centimeter paper, proving that they have a thin waist. A thin waist is often considered a symbol (象征) of beauty.