When three professors are seated in a restaurant, the hostess asks them: "Does everyone want coffee?

When three professors are seated in a restaurant, the hostessasks them: "Does everyone want coffee?" The firstprofessor says: "I do not know." The second professorthen says: "I do not know." Finally, the third professorsays: "No, not everyonewants coffee." The hostess comesback and gives coffee to the professors who want it. Howdid she figure out who wanted coffee?