The Muppet Show: Time in a Bottle

I watched this many times on a Muppet Special I had on VHS that was essentially a clip show. One of my all time favorites.
An unnamed scientist conducts self experiments with rejuvenating drugs in his laboratory. Jim Henson voices the line of Muppets who drink the series of different potions and becomes younger after each verse until he reverted to his initial age to sing the final line of the song. He changes his voice gradually on each verse from the gruff old man to the fresh faced young chemist, delivering the last line in a melancholy and familiar tone. Just wonderful.

From episode 31 of “The Muppet Show” with special guest star Edgar Bergen.