(Don't worry — you won't need a credit card)
Lesson One: I've always believed that a cartoon caption is funnier if it is posed in the form of a question — if possible — rather than in the form of a declarative statement. I feel that the question format effectively invites the reader to participate in the humor, perhaps even prompting the reader to silently but knowingly answer the question posed.
I realize that this is somewhat like what Neil Simon has one of his characters saying in one of his plays — the old comedian who adamantly insists, and I'm paraphrasing here, "You know what's funny? Words with a 'k' are funny — 'pickles' is a funny word."
Well, pickles IS a funny word, isn't it?? Notice how that last question got you involved?
End of Lesson One, and probably the end of the Eli Stein School of Gagwriting.