Talking about assignments helps your teenager think about them in a more critical fashion.
Examples:
Chemistry: "I never really understood the periodic table of elements. How does it work?"
Middle and High School Literature: "I never read ______ in school. What is it about?" or "I remember reading that in school. What did you think of the main character? She always reminded me of Aunt Mildred."
Math: "How is that math used in real life?"
Social Science: "How could you turn that idea into a business or money-making venture?"
Writing: "With texting, blogs, and social networking, writing seems to be back in style. Do you think all this online writing helps improve writing skills or does it hinder them? Where do you think you'll do the most writing after school is over? Do you find you write differently in different places?"
Foreign language: "Could you teach me the phrases I would need to know if I were to visit Paris?" or "How is German grammar different from English grammar?"
History: "Do you think we could learn anything today from how the average German citizens responded to *censor*? Do you think we could be fooled by propaganda and charisma the way they were then?"