What is a mirror?

Is it simply a reflective surface in which we check our appearance, is it somewhere to sing and rejoice "Mirror mirror on the wall... Who IS the fairest of them all?" Who is the fairest? Who made this up? Who is "all"... deep and frankly disturbing প্রশ্ন we raise here, friends. Is it just a piece of glass that reassures us or... bigger yet.. hold it, friends.. a door to a parallel universe???? Sorry to stop on such a shoking and intruging প্রশ্ন but it's just too much. I have to lay down.