In "The Difficult Path," how does Lingsi learn to read?

When Lingsi was sold to the Li family, Mrs. Li promised her mother she would teach the child to read before she was six. However, she promptly forgot about the promise. On Lingsi's sixth birthday, however, Mr. Li could not get the family incense to light. Convinced there must be some curse upon it, he consulted his wife, who admitted she had not done as she had promised. As a result, Mr. Li ordered that Lingsi be taught to read alongside her adopted brother FuDing.