South Park: Season 2, Episode 17

2 - 17: Gnomes (a.k.a. Underpants Gnomes)

The kids must give a presentation on current events to prove that Mr. Garrison has actually taught them something. The boys are assigned to work with Tweek, who is sure that gnomes are stealing his underpants. Meanwhile, Tweek's dad uses their project to promote his own personal cause.