The following true story will add to the enjoyment of the invented joke it contains.
Years ago my brother (who is on these boards as Fish) took a student trip to the Far East. As I recall, he spent most of a month in China, about a week in Thailand, and a few hours in Japan (transferring flights). Before leaving, he talked about how much he was going to miss us, so I conceived what I thought would be a pretty cool going-away present: I spent several hours writing something like twenty short letters, then sealing them in dated envelopes he was to open over the course of his trip, one per day or every other day (I don’t recall the specifics). Most of them were pretty silly, just to give him a chuckle at the beginning of his day, or whatever. One of them, for example, was a sketch of fireworks, to be opened on the Fourth of July, which he would be spending at a U.S. embassy. On another one, I drew a little dark blotch, and added an arrow and a label: “dead bug.” This turned out to be extra hilarious, because apparently, that morning, shortly before opening the envelope, he and his roommate had spent some time killing Chinese cockroaches in their bathtub. 
Okay, so, the fourth or fifth letter said, basically, this:
*Here’s a riddle for you! Why is Lincoln’s head on the penny?
(Answer next envelope.)*
The next letter started out with something completely unrelated to the answer to the riddle, and then there was a P.S.:
Sorry! I forgot to include the answer to the riddle in this envelope! I’ll do it next time.
The next letter did the same thing. Unrelated stuff, and “sorry, I forgot.”
Because, see, the thing is, I had no idea what the punchline would be. When I wrote the question, I had no answer. I just thought it would be funny to draw it out as long as I could, “forgetting” and promising in the next one. I really had no idea how I was going to resolve the joke, whenever I got around to it.
But then, after writing four or five of those, and keeping the unanswered riddle on a mental back burner, a light bulb went on, and I had my punchline.
So several envelopes later, I wrote something like:
*Because if it was his feet, you wouldn’t know who it was!
Now wasn’t that worth the wait?*
The point being, of course, that it wasn’t. Which made it extra funny.
(I love this story. :))
Years later, I resurrected the sequence-of-envelopes gift for a friend who was taking an extended trip to Russia and Ukraine, but as it turned out, she found herself feeling horribly lonely in the Kiev hotel room, so she opened all the envelopes at once, and then cut her trip short. 
Still a pretty good going-away gift, though, I think.