I am a cognitive scientist and my new book is called "When Everyone Knows That Everyone Knows, Common Knowledge and the Mysteries of Money, Power and Everyday Life. Common knowledge is interesting because it's what ratifies our social relationships, our relationships of friendship or deference or love or transactional relationship. A good example of common knowledge are channels for nonverbal communication that have often puzzled people, psychologists, philosophers, for centuries. Also, the thing about social media is that because of the like and repost buttons, anything could go viral. So it opens up a whole new dynamic of common knowledge, or at least the perception of common knowledge.