Karkaroff hesitated for a second, but the temptation of Felix Felicis was too great. He picked up the bottle of golden liquid, unscrewed the stopper, and tipped his head back to drain it.
The golden liquid slid down his throat, and Karkaroff's eyes instantly lit up. He took a deep breath, looking as though he had grown ten years younger. Even the wrinkles on his face relaxed a little.
"Wonderful," he murmured, savoring that marvelous sense that he could do anything. "Absolutely wonderful..."
Will watched his reaction, the corners of his mouth lifting slightly. "Do you like it?"
"Of course I do!" Karkaroff said excitedly. Then, realizing he had lost his composure, he cleared his throat and tried to recover his dignity as headmaster. "What I mean is... well, it's quite good. Very good indeed."
"I'll let you have it regularly from now on," Will said.
Karkaroff's eyes lit up again. "Regularly?"
"Yes. I can give you a bottle every few days, or even more. All you have to do is cooperate."
Karkaroff's smile stiffened for a moment. He eyed Will warily. "Cooperate with what?"
Will did not beat around the bush. "I want to experiment on you."
Karkaroff froze.
"Experiment?" he repeated, a hint of coldness entering his voice. "What sort of experiment?"
"An experiment in Potion Transfiguration," Will said. "You drink Felix Felicis, I observe your condition, and then I run a few... tests on the Felix Charm I'm going to use for the transfiguration."
Karkaroff's expression darkened. He stood, braced both hands on the desk, and looked down at Will.
"Mr. Behrens," he said in a low, threatening voice, "have you misunderstood something? I'm the headmaster of Durmstrang, not a Guinea Pig for your experiments."
Will's expression did not change. "I know."
"No, clearly you don't." Karkaroff