Please answer the following question: Generate a question about the following movie plot: In nineteenth century China, two girls named Snow Flower (Gianna Jun) and Lily (Li Bing Bing) are forever bonded together as sworn sisters. They are paired as laotong by a matchmaker who is also responsible for arranging their marriages. They are isolated by their families and communicate by writing in a secret sisterly language, Nu shu (a historical practice in China in that period) on a unique Chinese fan that Snow Flower possesses. Meanwhile, in the present day Shanghai, their descendants Sophia Liao and Nina Wei struggle with the intimacy of their own pure and intense childhood friendship. As teenagers, Sophia and Nina were introduced to the idea of laotong, and they signed a traditional laotong contract on the cover of Canto-pop Faye Wong's album Fu Zao (Restless in English). Faye Wong was their favorite singer and their liberated dancing to the "degenerate" sounds of the cheerful refrain "la cha bor" was one of the reasons Sophia's stepmother attempted to separate them. Eventually they are separated but come together again when Sophia falls into a coma after being struck by a taxi while cycling. Reunited at long last, they must come to understand the story of the strong and close ancestral connection hidden from them in the folds of the antique white silk fan or lose one another forever in the process.
Answer:
What do the girls write on?