This article is from the alt.usage.english FAQ, by Mark Israel firstname.lastname@example.org with numerous contributions by others.
The original version of this joke was apparently by Mark Twain,
and can be found at
<http://comedy.clari.net/rhf/jokes/87/2094.10.html>. A more
elaborate version, by Dolton Edwards (probably a pseudonym), which
gives no credit to Twain, was published in the science fiction
magazine "Astounding" in 1946 and can be found at
Various other versions are in circulation.