HOW A MIRROR FORMS IMAGES
Basically, anything with a smooth surface that reflects almost all of the light that hits it — with only very little light absorbed or scattered — can be a mirror. The key factor is a smooth surface, because rough surfaces scatter light instead of reflecting it.
When photons — rays of light — coming from an object (your smiling face, for example) strike the smooth surface of a mirror, they bounce back at the same angle. Your eyes see these reflected photons as a mirror image.
Of course, not all smooth surfaces act as mirrors. If a smooth surface absorbs the photons, they can’t bounce back and there will be no reflection.