UNDATED (AP) -- "The Simpsons" airs its 500th episode this Sunday. It's only the third primetime scripted series to have reached that milestone. The others are "Gunsmoke," which holds the record with 635 episodes and "Lassie," which was on for 588 episodes. Producer Mike Reiss tells USA Today that The Simpsons "could go on forever." He says it's a show "about the world," so they'll run out of material "the day the world stops being interesting."