A Venezuelan and a Brazilian are seated next to each other on an airplane, and the two manage to make small talk for the duration of the flight, even though they claim to speak different languages. Likewise, a person from Shanghai and another from Hong Kong are seated next to each other but cannot make themselves understood, even though they both claim to speak Chinese. Common sense suggests that two people speak the same language if they can communicate with each other and different languages if they cannot. However, linguist Tore Janson, in his 2002 book "Speak: A Short History of Languages," maintains that mutually intelligibility is not the best test of whether two modes of speech are the same or different languages. Rather, the key is the speakers' own attitudes about the language they use.