Without applications of physics such as counter-weighted sets and backdrops, inclined planes, stage lighting instruments, and other mechanisms for "deus ex machina," dramatic productions would revert to the words only--fine for Shakespeare and Becket, but not good for audiences who are accustomed to experiencing plays with the eye as well as the ear. Pepper's Ghost is a 19th-century stage illusion, based on basic optical principles, that can find its way into your introductory classroom.