At what point in the Shirley Jackson short story "Charles" did you figure out the ending? What led you to this conclusion at this point?

I was actually pretty clueless before the surprise ending
was revealed by Shirley Jackson in her short story, "Charles." I wondered about the
imaginary Charles and why he had never been seen by anyone but Laurie. After a
rereading, the clues given by the author become more obvious. Laurie's increasingly bad
behavior at home should have been one clue of the bad habits he was picking up in
kindergarten. The fact that Laurie seems to enjoy telling of Charles' exploits is
another clue. Laurie's laughing "insanely" should certainly be yet another clue that the
young boy was in need of some counseling, and that Laurie may actually be suffering from
a split-personality disorder. Since the story was told from the viewpoint of the mother,
who still saw Laurie as her innocent, little boy, it is no wonder that the ending was so
surprising to most readers.