Kenneth H. Rosen, Chapter 1

Question

When three professors are seated in a restaurant, the hostess asks them: “Does everyone want coffee?” The first professor says: “I do not know.” The second professor then says: “I do not know.” Finally, the third professor says: “No, not everyone wants coffee.” The hostess comes back and gives coffee to the professors who want it. How did she figure out who wanted coffee?

Answer 1

The third professor does not want coffee. The question asked is does everyone want coffee? The first two professors answered I do not know because they want coffee but don't know what the other professors wants. If they did not want coffee they would answer No, not everyone wants coffee, like the third professor.