From Muppet Wiki

Letter I (Beep Beep) is an animated segment/song about the words that start with the short sound of the letter I. In this segment, a man drives in a little bathtub-like car with eyes and mouth; as he moves along, the cartoon illustrates some examples of short-I words in the song. (When the lyrics mention "illegal", for instance, a policeman stops the driver for driving in the wrong direction.) The segment ends with the man taking a lower-case I into his car as a passenger.