Humor must not professedly teach, and it must not professedly preach, but it must do both if it would live forever. By forever, I mean thirty years.

If you would have your work last forever, and by forever I mean fifty years, it must neither overtly preach nor overtly teach, but it must covertly preach and covertly teach. | Attributed to Twain by J. Michael Straczynski in The complete book of scriptwriting, 2002, Writer's Digest Books