Professor Snape has hidden this magic mirror in his closet in order to prevent students from getting into trouble. Hey! What are you doing in here? You must be a Gryffindor! Well, if you insist upon knowing how it works, simply click your wand upon the mirror and it will reveal your deepest heart's desire in the window below it. Rumor has it that Peeves has been knocking the thing about, however, so if the mirror seems to be incorrect, go ahead and click again.