Common knowledge: Knowledge that not only you know, but that you know that others know.
One example of the power of common knowledge is goods associated with luxury and high status. By buying and prominently displaying such goods you can signal your status to others.
Luxury cars like Lamborgini rely on the common knowledge of their status to drive sales. You don't buy a Lamborgini just because you think it's a nice car. You buy it because other people think it's a nice car. A lesser known super car might not be as valuable to you because the car's price and status aren't common knowledge.