A beer for whoever can answer this one: A traveller visits an island inhabited by two types of people, knights and knaves. Knights always tell the truth; knaves always lie. The visitor falls in love with a local girl and wants to marry her. But before marrying he wants to be sure she is not a knave. An island tradition prohibits men from speaking to women until they are married. So the traveller must ask the girl's brother, who may be a knight or a knave and is not necessarily the same type as his sister. The traveller is allowed to ask the brother one question to find out if his potential bride-to-be is a knave. What is the question?