"Mister Chuckle Teeth" is the theme song of the fictional "Mister Chuckle Teeth" children's television show from the episode "Familiar" of the television series The X-Files. It features the show's possessed mascot, Mister Chuckle Teeth, opening his in-universe television program.
The song was performed by a children's chorus.
The song itself is later utilized by Mister Chuckle Teeth to manifest and attack one of his victims, where he takes on a truly demonic visage and threatens to drag them to Hell - this is the only time the song is actually "sung" by the demon itself, rather than just being a creepy theme song for the fictional show itself.
Mister Chuckle Teeth, happy as can be
Mister Chuckle Teeth, won't you play with me?
When I am feeling lonely, lonely as can be
Oh Mister Chuckle Teeth, won't you play with me?