Copyright 2009 Richard Termine / Sesame Workshop

"Sesame Street" celebrates its 40th birthday when First Lady Michelle Obama drops by and teaches the street's residents about the many benefits of planting a garden and healthy eating on PBS Kids today, the exact day the series debuted on TV four decades ago. ET also takes a look back over the years at other stars who have visited -- and have sung -- with Elmo, Big Bird and the Cookie Monster.