You could execute it and find out.

Assume that the program produces no messages whatsoever. You run it without parameters, it says nothing. You pass a file as parameter, it says nothing. You list the directory, there's no new file, the one passed hasn't changed. Hmm, that probably wasn't it.

Or suppose you have a library with absolutely no documentation.

Tell me how you would go about finding out what the code does at this point.

Of course the fact that code is executable by a computer is not quite incidental, but the essence of that saying is that you should write your code as if it were. In fact, you are already thinking by these lines, even if you don't realize it.

Makeshifts last the longest.