In his story, "The Green Door," O. Henry employs hyperbole, or obvious exaggeration, for humorous effect. In defining true adventurers, O. Henry writes that the Prodigal Son is a "fine example--when he started back home." Then, he describes Rudolf Steiner as a "true adventurer," explaining that there are few evenings that he did not go forth from his "hall bedchamber in search of the unexpeted and the egregious." However, since Rudolf simply looks for adventure in his immediate area, his actions seem somewhat less than daring.
Paradox is an apparent contradiction--a statement that, while appearing to be contradictory reveals a kind of truth. Like other writers, it seems that O. Henry uses paradox to express the complexity of life by showing how opposing ideas can be both contradictory and true. Rudolf Steiner, "the adventurous piano salesman," enters the building where he conceives that "his adventure must lie" and discovers behind the green door a starving young shopgirl. After becoming enamored of her, Rudolf fetches food and hot tea for her; they dine together and, the Romantic that he is, Rudolf becomes jealous at the idea that someone else might have come to her door. Also, he lies about why he has come to her door so that she will not know about the card that has directed him to her"strange expedient." As he shuts the door, ironically, Rudolf notices that there are several green doors, and then learns that the Negro has been handing out cards advertising a play in town that is called, "The Green Door."
While Rudolf is not adventurous enough to learn what the play is about or to check the entrances behind the other green doors, he is certainly romantic enough to believe that is was Fate "that doped out the way for me to find her." So, the truth of the paradox, or apparent contradiction, is that Rudolf is adventurous enough to embark upon a romance with the pretty little shopgirl.