Magic mirror
In short: The object that answers the queen’s questions in Snow White.
Object
How does it work in the tale?
The mirror tells the queen something beyond what she sees in her reflection. The question stays the same, but a different answer sets her next action in motion.
How does the mirror change the scene?
A new answer tells the queen that Snow White is alive and where she is. Once the question recurs, the reader can anticipate that the answer will set another trap in motion.