The underpants gnomes sneak into Tweek's room at night and steal his underwear.
Added:

South Park Season 2 Episode 17 Quotes

Time to go to work, work all night, search for underpants yay, we won't stop till we have underpants yum tum yummy tum yay!

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Stan: Okay we have to do this stupid report so... (Tweek freaks out) So let's figure out what to do it about.
(long pause)
Cartman: How about we do it on that Raymond guy on TV, you know everyone loves Raymond.
Kyle: No, Cartman, we can't do it on Raymond again. It has to be on a current event in South Park.