Sometimes, you plan your lesson backwards. You find something you really want to use in class and so you plan your lesson around it, even though it doesn't really fit in with where you're up to on the curriculum. Maybe it's a song. Maybe it's a great text. Maybe a video clip off the internet, or some new technological innovation.
This is a short story which illustrates how too much extrinsic motivation can kill off intrinsic motivation. Bear this in mind when you try to get your students' attention with lines like, "Pay attention - this is in the exam!". If you do this, you may be killing off any intrinsic interest they might have had for the stuff you are asking them to pay attention to.