Page "Distributed knowledge" Paragraph 1

The logicians Alice and Bob are sitting in their dark office wondering whether or not it is raining outside.

Now, none of them actually knows, but Alice knows something about her friend Carol, namely that Carol wears her red coat only if it is raining.

Bob does not know this, but he just saw Carol, and noticed that she was wearing her red coat.

Even though none of them knows whether or not it is raining, it is distributed knowledge amongst them that it is raining.

If either one of them tells the other what they know, it will be clear to the other that it is raining.