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 PratchettMort was already aware that love made you feel hot and cold and cruel and weak, but he hadn't realized that it could make you stupid.
Terry PratchettThe duke had a mind that ticked like a clock and, like a clock, it regularly went cuckoo.
Terry Pratchett