A man and his wife invite 5 other couples to a dinner party. As the guests arrive to visit before dinner, they shake hands. Not everybody shakes everybody's hands, and of course, no one shakes hands with his own spouse. Later, as they sit down to dinner, the host asks each other person, including his wife, "How many hands did you shake?" He notices, to his surprise, that each respondent shook a different number of hands. How many did his wife shake?