What is the main theme of "The Open Window" by Saki?

One
important theme of this satirical
short story is revealed in the symbolism of the characters' names.
There is Framton Nuttel, Mrs. Sappleton and the niece, who is both a "very self-possessed
young lady of fifteen" and a "child." Let's look at these and find the theme
alluded to.

The name Framton indicates someone
who is reclusive and aloof and given to worry and mental tension. This sounds exactly like the
characterpresents. The surname Nuttel (Nuttall) is a place name
indicating someone who lives in and tends a nut grove. "Nut" is also slang for a
foolish person or a person who is thought mentally unstable.

Sappleton appears to be a made-up surname derived from the
Old English 's¦p' that has come to be slang for someone who is foolish and gullible. This
perfectly describes Mrs Sappleton in regards to her manipulative niece.

We
learn the niece's name is Vera, ironically meaning "faith" and "truth." She
is the pivotal character, and we can deduce something about her from her descriptive tags. She
has the appearance of being a mature and trustworthy young lady as she is
"very self-possessed" yet the narrator calls her a
"child" when she begins her tale:

"Her greathappened just three years ago," said the child;
"that would be since your sister's time."

The
dominant theme that emerges from the satirically symbolic character
names and this satirical description is that of what foolish, gullible and mentally strained
people do to themselves and allow to be done to others. The mentally stressed "nut"
allows himself to be manipulated and tormented, even by his sister who insisted he visit this
part of the country. The foolish and gullible "sap" allows herself to be fooled and
tricked by her niece and becomes an accomplice to the harm her remorseless mischief causes to
innocent people (one must suppose this is a past-time of long standing with the self-possessed
niece). Thus the theme is the harm done by foolish and unstable
persons to themselves and to others.

In addition, the niece introduces the
equally strong theme of appearances versus reality. Her name
indicates this theme since she is not very faithful to the truth in what she tells Framton or
her aunt. While she appears to be a self-possessed young lady, she is in reality a mischievous
child, just as while the approaching figures appear to Nuttel to be ghosts, they are in reality
the returning husband and brothers.

In the deepening
twilight three figures were walking across the lawn towards the window, they all carried guns
under their arms, and one of them was additionally burdened with a white coat hung over his
shoulders. A tired brown spaniel kept close at their heels.