Once upon a time, there were a knigh and a witch. They both lived incastles. The knight's castle was big and full of expensive stuff. While the witch's castle was small, and was on th point of caving in. On one day the knight decided to visit the witch, so he walked out of his castle, heading towards the witch's castle. He knocked on the door, and after a while, the witch opened the door. She told him to come in and have some tea. So the knight came in and drank a cup of tea with the witch. When he wanted to return to his castle, he passed a coridoor on his way back, still inside the witch's castle. Curiously, he peered inside and saw a mirror. But it didn't reflect only himself as a normal mirror would do, it reflected him... and his mother and father (who has died a few years before). The knight couldn't believe his eyes. He looked all around the room, but couldn't see anyone. Then he looked back inside the mirror, and there they were! The knight, lost in trance, stood there staring at the mirror for at least half an hour. Deciding it was time to go back to his own castle, het went back. But from that point on, he'd visit the witch's castle at least one time a day, drink tea with her, and then go back to the mirror to stare at it for about an hour, just to see his parents again.
On a certain day, he offered the witch 100 euro's for the mirror. The witch, totally perplex, asked him why he wanted to pay so much for such an old mirror. The knight, thinking it would sound to childish to say that he missed his parents and that he wanted to see them more often, said that the mirror would fit in well with his other possesions he had stored inside his castle. The witch refused his offer. So still he had to go to the witch every day, to look at his parents inside the mirror. But on one day, he was getting sick of having to listen to the witch's talking an having to drink tea every time. So he decided t osteal the mirror. All went as planned, and the knight ran out of the witch's castle clutching the mirror in his hands. He hung it on a wall in his bedroom, and started to stare at it again. But his parents weren't looking back at him anymore. In stead, and to his horror, he saw the witch staring back at him out of the mirror! He asked her why he didn't see his parent's anymore inside the mirror. The witch (though he hadn't expected it) answered him. “Because this mirror was taken from me, without my permission. If you had just told me honestly what your intentions were, I would have sold it to you, or even given it as a present.” He asked her how he could see his parents again, and she anwsered. “As long as I exist, I will make sure you never see them again inside this mirror. That, is your punishment for not being honest with me and stealing this mirror”. “As long as she existed...” The knight made up his mind in a split-second. Blowing all cautions to the wind, he stromed of to the witches castle, a knife clutched in his fist. Whenb he arrived he banged on the door, and when the witch opened the door, he stabbed her in the chest. Not paying a second thought to the witch, he ran back to his castle to look inside the mirror again. To his relief, he saw his parents staring back at him again. But they weren't smiling... They looked sad. Sad that their son had become a murderer. After a few hours of staring into the mirror, looking away again, and staring back hoping his parents were smiling again... He took the knife, and killed himself, our of pure misery.

Messages behind this story:
Don't steal from others (he stole the mirror but didn't get what he wanted).
Be honest (he didn't get what he wanted because he had hidden his true intentions).
Don't be afraid what others may think of you if you tell them the truth (he hadn't revealed his true intentions, so hadn't got what he wanted).
Don't live inside your dreams (the knight was living inside of his dreams, and therefore forgot to be a good person in the real world, again getting his punishment for his actions).

Little clarification:
The mirror shows whatever the person staring inside it desires most, in the case of the knight it was his parents, who had dies a few years before, but only to those who deserve it most.
The concept of the mirror comes from “Harry Potter and the Philosopher's Stone”, there it's called “The Mirror Of Errised”. But I worked out the idea a bit differently.