"Tropic Thunder" star Jack Black appeared with Elmo on US television to help teach children about octagons. Black, who also starred in "Shallow Hal" and "Kung Fu Panda", quite happily over-acted through his Sesame Street segment, searching desperately for an octagon for his educational cameo until rescued by the red-furred Elmo, who had a "Stop" sign to hand.

Comic actor Jack Black has been very busy in recent years. He was nominated for a Golden Globe for his role as a fake music teacher in "School of Rock" and keeps himself busy in his spare time as the lead singer of comedy rock band Tenacious D. His next film will be the comedy "The Year One", scheduled for release on June 5, 2009.