"Last week's controversy about Katy Perry's Sesame Street guest appearance reminded us that the show has a long history of working with celebrities. And while cleavage isn't exactly what they're looking for, Sesame Street's producers have always embraced clever adult humor. We dug through the archives to find some of the smartest, funniest, and strangest celebrity appearances in Sesame Street history, from Richard Pryor to Ricky Gervais.