It's an old magical principle - it's even filtered down into RPG systems - that magic, while taking a lot of effort, can be 'stored' - in a staff, for example. No doubt a wizard spends a little time each day charging up his staff, although you go blind if you do it too much, of course.
Terry PratchettOh dear me, the Booker people don't like me. I don't care. I have been given an award for being taken not seriously, and I am very, very pleased about that.
Terry Pratchett