The Grim Adventures of Billy and Mandy - Season 4 - Eps 19: Runaway Pants
At school it is time for the "Presidents Fitness Test", but Billy is absolutely unfit for exercise. On the other hand, Nergal Junior uses his shape-shifting powers to become fit at will and excel in every test. Billy asks Junior to transform into a pair of pants that he can wear, so he can run fast and win the 50-meter race. Since Junior has never attempted to transform into inanimate objects he cannot stop running and cannot revert to his human form. Billy keeps running forever while he becomes a national sensation known as the "Fitness Lad". It is revealed that the story was just all a fabrication created by the narrator, who parodies The Twilight Zone.
About The Grim Adventures of Billy and Mandy

Title: The Grim Adventures of Billy and Mandy
First Air Date: 2001-08-24
Last Air Date: 2007-11-09
Status: Ended
Rating: 8.045/10 (from 958 votes)
Language: EN
Seasons: 8
Total Episodes: 184
Network: Cartoon Network
Genres: Animation, Comedy, Kids
Production Companies: Cartoon Network Studios
Synopsis
The exploits of the Grim Reaper, who has been forced into being the best friend of two children. A spin-off of the show Grim & Evil.
Cast

Greg Eagles
Grim (voice)

Richard Steven Horvitz
Billy (voice)

Grey DeLisle
Mandy (voice)

Vanessa Marshall
Irwin (voice)