Shaggy-dog stories originally were merely long, pointless
stories.  Eventually they became long stories in which the conclusion
was a pun.

        The original shaggy-dog story--from which the genre derives
its name--involves the little boy who enters his dog in the local
shaggy-dog contest.  The first judge says, "Myyyy, what a shaggy
dog!"  The second says, "Ooooh, what a shaggy dog!"  And the third
says "Graaaacious, what a shaaaaggy dog!"  Similar occurrence at the
county, the district, the state, the regional, national, and hemispheric
shaggy-dog contests, with identical judicial comments, embellished
with gusto [and perhaps picante sauce, if you have some handy].  However,
at the world shaggy-dog contest, the third judge says [say this with
flat intonation], "I don't think that's such a shaggy dog."

        Pointless, right?

        But shaggy-dog stories developed into vehicles for bad
puns.  Or should I say "good" puns?  Difficult to say, since the
better a pun is, the louder the audience groans.  What follows is a
more modern shaggy-dog tale.  I shall give it in abbreviated form;
when you inflict it upon others, you should stretch it out till their
patience is thin.