Ricky Gervais has teamed up with Elmo to mark the 40th anniversary of the popular children's television show Sesame Street.In the episode, the British funnyman sings the three-year-old monster a celebrity lullaby to help him nod off.What is a celebrity lullaby?"That's when an unbelievably famous person comes over and sings to you," Gervais quips.Throughout the clip, which airs in the US on November 18, Gervais jokes that he agreed to appear on the show before he found out he wasn't getting paid. At times Elmo breaks into uncontrollable laughter.Out-takes of the interview, in which Gervais broaches the the topic of drugs and necrophilia, were posted online earlier this year and became an instant hit.WATCH THE OUT-TAKES: What the children won't see