"Detention!" The Potion Master snapped automatically. "This is your O.W.L. year, all students will either succeed or fail on their own. I will not allow anyone to tamper with someone else's brew. No matter if the tempering is harm- or helpful. Is that understood, Mr. ... Potter?" Only at the end of his speech did the man realise who he had criticised so angrily. Still, he drew himself up to full height and glared.