The situation with Slughorn was different; you always had the ability to persuade him, you just needed to tweak the circumstances a bit.
Unorthodox, but what a stroke of inspiration, Harry, of course, that would tend to counterbalance the occa-sional side effects of excessive singing and nose-tweaking.
He suddenly remembered Dumbledore’s idea of a few words: ’nitwit’, ’oddment’, ’blubber’ and ’tweak 1, and again, had to suppress a grin … what was the matter with him?
All in all, the temptation to take another gulp of Felix Felicis was becoming stronger by the day, for surely this was a case for, as Hermione put it, "tweaking the circumstances"?
There are no more uses of "tweak" in the book.
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She was completely unaware that with one twist of the tongue she could plunge Jean Louise into a moral turmoil by making her niece doubt her own motives and best intentions, by tweaking the protestant, philistine strings of Jean Louise’s conscience until they vibrated like a spectral zither.