Your pupil changes size to control how much light enters your eye. You can observe that the pupil of your eye changes size in response to changes in lighting. You can also experiment to determine how light shining in one eye affects the size of the pupil in your other eye.
Plastic mirrors are safer than glass and are available at plastics stores.
No assembly is necessary. Place the magnifying glass on the surface of the mirror. Look into the center of the magnifying glass with one eye. If you wear contact lenses or glasses, you may either leave them on or remove them. Adjust your distance from the mirror until you see a sharply focused and enlarged image of your eye.