My favorite exchange ever in the show is when Zim is trying to test Gir's new guidance system, and they end up getting hopelessly lost in the city. When Zim tells Gir to use the guidance chip to get them home...

GIR: Oh, I left it at home.
ZIM: You left what at home?
GIR: The guidy... Chippy... Thingy.
ZIM: Why would you do that?!
GIR: To make room for the cupcake!

...after which a cupcake pops out of Girs head.