I came to the topic of common knowledge, that is, everyone knowing that everyone knows that everyone knows something, through my interest in language. In language, most of what we say isn't exactly what we mean. People say things like, if you could pass the salt, that would be awesome. And that's not literally what they mean. They mean, give me the salt. And many of our conversations use euphemism, innuendo, and beating around the bush. How come? Why don't we just blurt what's on our mind? I realized that the answer is that language has to do two things at once. On the one hand, you're conveying a message. The other hand, you are negotiating what kind of social relationship you have. Are you friends? Are you lovers? Are you transaction partners? And that is done through common knowledge. That is, two people are friends if each one knows that the other one knows that the first one knows that the second one knows that they're friends. And so when we use euphemism, I realize what we're doing is we're trying to prevent common knowledge. And that's what led me to the realization that common knowledge is the key to all of our social relationships.