to?

He tried to edge to the left, to get in front of the glass without
Quirrell noticing, but the ropes around his ankles were too tight: he tripped and fell over. Quirrell ignored him. He was still talking to himself. "What does this mirror do? How does it work? Help me, Master!"

And to Harry's horror, a voice answered, and the voice seemed to come from Quirrell himself

"Use the boy... Use the boy..."

"What the fuck was what?" yelled Amelia. 

Quirrell rounded on Harry.

"Yes -- Potter -- come here."

He clapped his hands once, and the ropes binding Harry fell off. Harry got slowly to his feet.

"Come here," Quirrell repeated. "Look in the mirror and tell me what you see."

Harry walked toward him.

I must lie, he thought desperately. I must look and lie about what I see, that's all.

Quirrell moved close behind him. Harry breathed in the funny smell that seemed to come from Quirrell's turban. He closed his eyes, stepped in front of the mirror, and opened them again.

He saw his reflection, pale and scared-looking at first. But a moment later, the reflection smiled at him. It put its hand into its pocket and pulled out a blood-red stone. It winked and put the Stone back in its pocket -- and as it did so, Harry felt something heavy drop into his real pocket. Somehow -- incredibly -- he'd gotten the Stone.

"Well?" said Quirrell impatiently. "What do you see?" Harry screwed up his courage.

"I see myself shaking hands with Dumbledore," he invented. "I -- I've won the house cup for Gryffindor."