Behind China's vaunted system of Internet censorship are throngs of specialized police officers, fake commentators, and ever-changing technologies. But China watchers have puzzled over the system's modus operandi. Some posts are swiftly culled, whereas others on seemingly more sensitive topics are left untouched. In the most revealing study yet of Chinese censorship, researchers at Harvard University describe in this week's issue of Science how they peered behind the curtain to find out what China's censors—and presumably the government officials operating behind the scenes—fear most: discussion of mass protests and other forms of collective action. The unprecedented participatory experiment on China's blogs, microblogs, and forums also flouts the conventional wisdom that the censorship apparatus is designed to squelch criticism of the Communist Party or its leaders.