Make the observation, in the midst of a great discussion of Act 2, Scene 4 of Measure for Measure (where Angelo propositions Isabella), that, if the scene is staged right, it's shocking and gross, yes. . . but also pretty hot.
If you do this, your students will gasp, look at you with wide, astonished eyes, and immediately stop talking.
But hey: I'm a big believer in creating my own gossip--and I can't claim that the comment was unpremeditated.