Chen Ailian, a 75-year-old dancer, leads a crowd of more than 100 middle-aged people on a square in the Sanlitun area of Beijing on Monday with her professional moves. It was said to be the largest so-called flash mob of middle-aged and elderly people ever to assemble in the city. The term refers to a crowd that comes together rapidly in response to notices on social media or by other means. Crowds gather for a specified activity and then dissolve as quickly as they came. [Photo/CFP]