How a program can reproduce its own text
A quine is a program that prints a copy of its own source code—the human-readable text that was written to make it run. The trick is not to have the program open its own file and copy it. In the classic puzzle, the program must generate its source from information already built into the program itself.
That sounds like a loop: the code must contain the text that prints the code, including the text that prints the text. A quine escapes the loop by dividing the job into two parts: a template and data that fills in the template. The program stores a representation of part of its own text, then combines that representation with the template to produce the complete source. When run, the result is an ordinary-looking copy of the program, not a mysterious duplicate created from nowhere.
Why the programming language matters
The details depend on how a language represents text. A string containing quotation marks may need escape characters, and the program must print those characters in exactly the right places. A working quine balances the code that performs the printing with the string data that describes that code; a tiny mismatch changes the output and breaks the self-copy.
That makes a quine a compact lesson in the difference between instructions and data: text can be both something a program acts on and something it describes. It is mainly a puzzle and teaching exercise, not a useful way to copy files. Its memorable payoff is that the printed text can be fed back to the same language and run again, producing itself once more.
