How is it safe to Transfigure a desk into a pig? Pigs breathe.
This should be safe if done briefly, right?
When it turns back into a table, we get bubbles in the table and sawdust in the air.
If a large amount of transfigured air has been incorporated into the bodies of bystanders (and pigs do breathe out oxygen, or at least humans do, because our lungs aren't terribly efficient), then we might have a problem.
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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