Answer: Vera's tendency for making up outlandish stories
The question refers to the story "The Open Window" by Saki (H. H. Munro).
In the story, Framton is visiting the house of Mrs. Stapleton. While waiting for her, Vera, her niece, tells her a story. It is not until the end that we realize the story Vera told is in fact a lie. In order to get out of the sitation she created, she tells her family another outlandish story, showing that this is a character trait of Vera, and not an isolated event. The fact that we do not know Vera's story is false until the end is vital for creating the desired effect of surprise on the reader.
