(0) I understood your joke
(1) I understood your joke, but I assume you do not, so be please allow me to explain it to you.
(2) I understood your joke, but did not find it to be sufficiently humorous to warrant my having read it.
(3) I did not understand your joke, but I believe that if had, it would have undoubtedly caused me great personal offense for reasons that you could not have anticipated. I therefore must challenge you to a duel.