Here's why:
* Cosmetic mirrors are typically concave mirrors. Concave mirrors can produce both real and virtual images, depending on the distance of the object (the girl's face) from the mirror.
* When an object is placed very close to a concave mirror (closer than the focal length), the image formed is:
* Virtual: Light rays do not actually converge to form the image; instead, our brains perceive the rays as coming from a point behind the mirror.
* Upright: The image is not inverted.
* Magnified: The image appears larger than the actual object.
Since a girl using a cosmetic mirror is close to the mirror and sees an enlarged, upright image, it must be a virtual image.