In physics this morning, the professor said, "It has to be in between. If you put it on top, it won't do anything. At that point the light is already dead. It's been killed. We can't resurrect dead light."  It was pretty funny until he said, "That's pretty funny.  I'd never thought of that one before."  If even you realize that your reused jokes aren't that good, you probably should stop using them.