Topics That Shock Classes

My favorite Doonesbury is this one.

My philosophy of science class last week made me think of it. Not because they are passionless note-scribblers, quite the opposite. It's a wonderfully lively bunch. But then we hit Hume's problem of induction and Goodman's grue paradox and they went absolutely bananas. Hume doesn't usually get the hackles up, but grue always does. Man, students HATE that argument...always gets them riled up.

My question is what else in your experience has this effect? What as a student just charged you up? What as a teacher do you find gets your students loud and passionate?