I strongly doubt that the quote that Joe posted (in the box above) is authentic. It doesn't sound like Mark Twain's style to me. It isn't particularly funny. Even when he wasn't writing for publication, Mark Twain was funnier than that. For example, while searching Twain's works for the word "grandfather" I found this (at Mark Twain's Letters, Complete, Arranged with Comment by Albert Bigelow Paine):

                                             21 Fifth Avenue, New York,
                                                 November 14, 1905.
DEAR MR. ROW,--That alleged portrait has a private history. Sarony was as much of an enthusiast about wild animals as he was about photography; and when Du Chaillu brought the first Gorilla to this country in 1819 he came to me in a fever of excitement and asked me if my father was of record and authentic. I said he was; then Sarony, without any abatement of his excitement asked if my grandfather also was of record and authentic. I said he was. Then Sarony, with still rising excitement and with joy added to it, said he had found my great grandfather in the person of the gorilla, and had recognized him at once by his resemblance to me. I was deeply hurt but did not reveal this, because I knew Sarony meant no offense for the gorilla had not done him any harm, and he was not a man who would say an unkind thing about a gorilla wantonly. I went with him to inspect the ancestor, and examined him from several points of view, without being able to detect anything more than a passing resemblance. "Wait," said Sarony with confidence, "let me show you." He borrowed my overcoat--and put it on the gorilla. The result was surprising. I saw that the gorilla while not looking distinctly like me was exactly what my great grand father would have looked like if I had had one. Sarony photographed the creature in that overcoat, and spread the picture about the world. It has remained spread about the world ever since. It turns up every week in some newspaper somewhere or other. It is not my favorite, but to my exasperation it is everybody else's. Do you think you could get it suppressed for me? I will pay the limit.
                              Sincerely yours,
                                        S. L. CLEMENS.

If the quote is authentic, it ought to be possible to find it in one of the works listed at the Online Books Page but it would take a lot of patience. You have to search one book at a time. Many, but not all of his works are searchable here.