Lucy in the Sky and other early performances
In the early days, when most moms stayed at home, there was a lot of adult humor and adult oriented music to get moms to watch with their kids. You'll find it in show #003, which first aired in the fall of 1969.
Lucy in the Sky was performed by the character who later became known as Prarie Dawn. At the time she was an "anything muppet". She performed a sweet, gentle version, with a chromakey kaleidescope pattern in the background. Then, on the chorus, she was joined by 4 wild, hippie-like anythingmuppets who screamed "Lucy in the Sky with Diamonds" at the top of their lungs, and disappeared to let Prarie continue.
In the same show you'll also find Bob dressed in a psychedelic shirt singing "Good Morning Starshine" from the musical "Hair" (which was later released on the "Songs from Sesame Street" album) and "Susan" singing, "I Enjoy Being a Girl" from the Rogers and Hammerstein musical "Flower Drum Song".