car and cdr are two basic commands in the Scheme language. Although pronunciations of cdr differ, our professor (who was from England) liked to say it "cudder." And since "cudder" rhymes with "butter," this was a pun just waiting to be made. Hence, this drawing.
As fate would have it, during my first semester as a PhD student at the University of Utah, I was called upon to be T.A. for CS 3520, a course very similar to BYU's CS 330. So I redrew my old Scheme comics (having long since lost the originals) in 2004 for the amusement of my students.