Granny Weatherwax was often angry. She considered it one of her strong points. Genuine anger was one of the world’s great creative forces. But you had to learn how to control it. That didn’t mean you let it trickle away. It meant you dammed it, carefully, let it develop a working head, let it drown whole valleys of the mind and then, just when the whole structure was about to collapse, opened a tiny pipeline at the base and let the iron-hard steam of wrath power the turbines of revenge.
— Terry Pratchett, Wyrd Sisters