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The first time I gave this quiz, in 2010, I was very surprised at the intensity of student responses in the discussion. Students are very concerned with where the entropy comes from in the case of free expansion, and have the feeling (although they wouldn't express it this way) that entropy should be conserved. The discussion took 30 minutes after the quiz was finished.

In 2011, I had a bit more time, and the discussion went longer–possibly *too* long. But it was very fruitful. I had talked previously about the Second Law, and about the entropy of a system plus its surroundings increasing, but somehow students don't grasp it, and this activity manages to bring out their concerns.