Кстати прямо по теме - ответ от Роулинг: http://www.accio-quote.org/articles/2000/0700-swns-alfie.htm

Q: It seems that the wizards and witches at Hogwarts are able to conjure up many things, such as food for the feasts, chairs and sleeping bags. . .if this is so, why does the wizarding world need money ? What are the limitations on the material objects you can conjure up ? It seems unnecessary that the Weasleys would be in such need of money. . . (Jan Campbell)
A: Very good question (well done, Jan!!). There is legislation about what you can conjure and what you can't. Something that you conjure out of thin air will not last. This is a rule I set down for myself early on. I love these logical questions!