"This edge," said Giacomo Paradisi, touching the steel with the handle of a spoon, "will cut through any material in the world. Look."
And he pressed the silver spoon against the blade. Will, holding the knife, felt only the slightest resistance as the tip of the spoon's handle fell to the table, cut clean off.
"The other edge," the old man went on, "is more subtle still. With it you can cut an opening out of this world altogether. Try it now. Do as I say — you are the bearer. You have to know. No one can teach you but me, and I have not much time left. Stand up and listen."

Ch. 8 : The Tower of the Angels - His Dark Materials - The Subtle Knife (1997)