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quine: /kwi:n/ [from the name of the logician Willard V. Quine, via
   Douglas Hofstadter] n. A program that generates a copy of
   its own source text as its complete output.  Devising the shortest
   possible quine in some given programming language is a common
   hackish amusement.  Here is one classic quine:

((lambda (x) (list x (list (quote quote) x))) (quote (lambda (x) (list x (list (quote quote) x)))))

This one works in LISP or Scheme. It's relatively easy to write quines in other languages such as Postscript which readily handle programs as data; much harder (and thus more challenging!) in languages like C which do not. Here is a classic C quine for ASCII machines:

char*f="char*f=%c%s%c;main(){printf(f,34,f,34,10);}%c"; main(){printf(f,34,f,34,10);}

For excruciatingly exact quinishness, remove the interior line breaks. Some infamous Obfuscated C Contest entries have been quines that reproduced in exotic ways.