What can/can't a wizard or witch conjour up?

[dungrollin]

"foodiedb29" wrote:
> I was wondering what sort of rules, laws, etc., govern what a wiz. 
or witch, can or cannot conjour up?  For example, why doesn't Ron's 
parents just conjour up money for their family?
Thanx all,
> David
>

Dungrollin:

Here are some pre-OotP posts from the main list which might interest 
you. It's a subject which gets discussed every so often, and there 
are plenty more hiding in the archives, so the following examples 
are by no means definitive.

CMC 6376 "The Limits on Magic" 
http://groups.yahoo.com/group/HPforGrownups/message/6376
Aberforth's Goat 7262 "Thermodynamics and Wizarding"
http://groups.yahoo.com/group/HPforGrownups/message/7262
Amandageist 11361 "Theory of magic food"
http://groups.yahoo.com/group/HPforGrownups/message/11361
Amy Z 20352 "Spells & Charms; the nature of magic"
http://groups.yahoo.com/group/HPforGrownups/message/20352

Hope that helps,
Dungrollin