Just reading the words The Simpsons might prompt the theme song to start looping in your mind. That's OK. Now you have a soundtrack for your efforts to recognize the celebrity cameos in these various Simpsons episodes. We may not know for certain where Springfield is located, but you should be able to use the image and a clue to remember the many famous folks who have appeared in the television series’ more than 600 episodes. Just remember - these celebrities weren't always performing as themselves. Or were they?