The Grim Adventures of Billy & Mandy

Runaway Pants (4x19)

:

At school, it's 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. Then, 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. However, since Junior has never attempted to transform into inanimate objects he can't stop running and can't revert to his human form. In consequence, Billy keeps running forever while he becomes a national sensation known as the "Fitness Lad". However, in the end it is revealed that the story was a lie created by the narrator, who parodies The Twilight Zone.