Conjuring Things Out of Thin Air

[cynthiaanncoe at home.com]

No: HPFGUIDX 28651

I can only think of four instances in which we know for certain that 
wizards conjure things out of thin air.  One is when Dumbledore draws 
a chair for Trelawney in PoA.  Another is when Dumbledore conjurs up 
sleeping bags for the students.  Another is when Sirius conjurs 
manacles for Lupin, Ron and Pettigrew.  Another is when Voldemort 
conjures the silver hand for Wormtail.

There are other instances of things appearing, such as when people 
cause ropes to fly out of their wand.  But the rope trick might not 
be pure conjuring, because the ropes fly out of the wand the way 
Molly's sauce pours out of her wand.  In the four examples of 
conjuring above, however, it seems that the item doesn't fly out of 
the wand, but kind of materializes out of thin air.  

Given that conjuring things out of thin air is a tremendously useful 
spell, it doesn't get used nearly as much as I would expect.  Ron 
could conjur dress robes in GoF, for instance.  Harry could conjure 
up some butterbeer rather than go to Hogsmeade.

Anyway, I was wondering what the limits of this might be.  One 
explanation from JKR (I think) is that things conjured don't last.  
That might not be the whole explanation, however, as I suspect 
Wormtail still has his silver hand.  And if Dumbledore's sleeping 
bags lasted all night, why shouldn't Ron's self-conjured dress robes 
last long enough for the Yule Ball?

All four instances of conjuring have something in common, though -- 
the person doing the conjuring is conjuring the item for the use of 
someone else.  So in the case of the manacles, the reason Sirius 
would conjur them would be that if Lupin did it, Lupin couldn't use 
them.  This might help explain why wizards aren't constantly 
conjuring things to meet their own desires -- they can only conjure 
things for others to use, and even then, only temporarily.  So maybe 
Harry could conjur dress robes for Ron if they ever go to another 
ball, but Harry would have to learn how to do this first, which may 
be a long time coming.

Anyway, does this make any sense, and have I missed any instances in 
which wizards conjure things from thin air?

Cindy