I read the sudoku analogy some time ago on stack overflow, only it was much shorter and concerning the question of time. How do you explain somebody why it is

I read the sudoku analogy some time ago on stack overflow, only it was much shorter and concerning the question of time. How do you explain somebody why it is hard to know when you will have accomplished a certain programming task? Just ask how long he will need for a sudoku, with a second in precision.