That's what you people never understand," said Rincewind,....

“ That's what you people never understand," said Rincewind, wearily. "You think magic is just something you can pick up and use like a, a -""Parsnip?" said Nijel."Wine Bottle?" said the Seriph."Something like that," said Rincewind cautiously, but rallied somewhat and went on, "But the truth is, is -""Not like that?""More like a wine bottle?" said the Seriph hopefully."Magic uses people," said Rincewind hurriedly. "It affects you as much as you affect it, sort of thing. You can't mess around with magical things without it affecting you. I just thought I'd better warn you.""Like a wine bottle," said Creosote, "that -""- drinks you back," said Rincewind. ”

Terry Pratchett

From : Sourcery