This is one of my favorite passages from HP and the Sorcerer's Stone:

"It shows us nothing more or less than the deepest, most desperate desire of our hearts. You , who have never known your family, see the mstanding around you, Ronald Weasley, who has always been overshadowed by his brothers, sees himself standing alone, the best of all of them. However, this mirror will give us neither knowledge or truth. Men have wasted away before it, entranced by what they have seen, or been driven mad, no knowing if what it shows is real or even possible.
The Mirror will be moved to a new home tomorrow, Harry, and I ask you not to go looking for it again. If you ever do run across it, you will now be prepared. It does not do to dwell on dreams and forget to live, remember that. Now, why don't you put that admirable cloak back on and get off to bed?"

Harry stood up.

"Sir- Professor Dumbledore? Can I ask you something?"

"Obviously, you've just done so, " Dumbledore smiled. "You may ask me one more thing, however."

"What do you see when you look in the mirror?"

(now this is my most favorite part...)

"I? I see myself holding a pair of thick, woolen socks."

Harry stared.

"One can never have enough socks, " said Dumbledore. "Another Chrismas has come and gone and I didn't get a single pair. People will insist on giving me books."