If you follow water issues long enough, especially in the West, you’ll hear or read: “Mark Twain said, ‘Whiskey is for drinking; water is for fighting over’.” And almost as often, someone else will say that Mark Twain probably didn’t say it, because no one has found it in his writings.
Finally, there’s a bit of new information on this topic. The publisher of Twainquotes.com, using Google Books, seems to have tracked it down to the 1960s. (Twain, of course, died in 1910.)
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