"Thank you, Severus," Dumbledore interrupted, turning back to Harry. "You may not know this, Harry, but goblin silver is prized for a very special property: it imbibes only that which strengthens it. That means that it never becomes dirty or tarnished." Dumbledore poured a few drops of the potion into the fuller; they smoked and hissed and quickly disappeared. "It also means that it absorbs useful qualities of the substances it’s exposed to. So this dagger will now put anyone cut by it into a deep magical sleep for one hour."