What is the irony and suspense in Saki's short story, "The Open Window"?

"MY aunt will be down presently, Mr. Nuttel," said a very
self-possessed young lady of fifteen; "in the meantime you must try and put up with
me."

Here, with this quotation, starts the suspense.
With 's simple opening come two important questions to the reader's mind. The first is how
self-possessed can a young lady of fifteen actually be? The second is why does she say
"must try and put up with me"? Our interest is piqued and our attention is fine-tuned
to see what peculiarities to come will answer the questions and explain her words.

The very next sentence introduces .

Framton
Nuttel endeavoured to say the correct something which should duly flatter the niece of the
moment without unduly discounting the aunt that was to come.

In addition to situational irony, the narrator's ironic tone is manifest. Ironic humor
is evident in the narrator's choice of using repetition to describe Nuttel's conversational
predicament: "duly flatter the niece ... unduly discounting the
aunt."

Both...