What could be easier than to write a program that produces its own code as output?

It’s not that easy – even a well-versed programmer will need to spend some hours to figure out the trick (which can be different for each programming language).

One trick is that the program must contain a description of itself, inside its own code. Because outputting the self-description also needs code, the self-description must be somehow compressed.

But once written, such a program --also called a "Quine"-- reproduces itself each time it is run, and this in turn is called a fixpoint. One could call this the fixpoint of life because any lifeform (including robots) must master this trick if they wanted to build themselves.