Harry Potter and the sorcerrer's stone

"So -- back again, Harry?"
Harry felt as though his insides had turned to ice. He looked behind
him. Sitting on one of the desks by the wall was none other than Albus
Dumbledore. Harry must have walked straight past him, so desperate to
get to the mirror he hadn't noticed him.
" -- I didn't see you, sir."
"Strange how nearsighted being invisible can make you," said Dumbledore,
and Harry was relieved to see that he was smiling.
"So," said Dumbledore, slipping off the desk to sit on the floor with
Harry, "you, like hundreds before you, have discovered the delights of
the Mirror of Erised."
"I didn't know it was called that, Sir."
"But I expect you've realized by now what it does?"
"It -- well -- it shows me my family --"
"And it showed your friend Ron himself as head boy."
"How did you know --?"

"I don't need a cloak to become invisible," said Dumbledore gently.
"Now, can you think what the Mirror of Erised shows us all?"
Harry shook his head.
"Let me explain. The happiest man on earth would be able to use the
Mirror of Erised like a normal mirror, that is, he would look into it
and see himself exactly as he is. Does that help?"
Harry thought. Then he said slowly, "It shows us what we want...
whatever we want..."
"Yes and no," said Dumbledore quietly. "It shows us nothing more or less
than the deepest, most desperate desire of our hearts. You, who have
never known your family, see them standing 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, not knowing if what it shows is
real or even possible.