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I was watching a Super Carlin Brothers youtube video when they touched briefly on the subject of Secret Keepers and the Fidelius Charm. They were confused over what info it could be used on. Their example was: if someone decided to entrust the knowledge of how to open doors to someone through the Fidelius Charm, would all people that were born after this have to be told by the person? Link to full video: The Sorting Hat's Big Secret

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    The Fidelius Charm is one of the few really big, gaping cases of clear deus ex machina in the Harry Potter series. It’s just consistently inconsistent, moving and changing at the speed of plot. If you search on this site, you’ll find lots of questions about the Fidelius Charm introducing both potential and real plot holes. – Janus Bahs Jacquet Aug 25 '18 at 23:44
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    I don't think there's anything in canon to contradict the obvious theory that the Fidelius charm can't affect common knowledge. Hiding a secret that is already known to a few people is a quite different order of magic to hiding a "secret" that literally everybody already knows. IMO, even the Elder Wand wouldn't be able to do that. I would imagine there are other factors, too, such as whether the people whose secret is being protected are willing participants or not. – Harry Johnston Aug 26 '18 at 0:26
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    @Janus, can you give us an example of an inconsistency? So far as I can recall there are only four good examples in canon - James and Lily, Phoenix HQ, Shell Cottage, and the Weasley home. With so few examples, it seems to me that there'd be a strict limit to how many inconsistencies the author could have introduced even if she'd been trying. :-) – Harry Johnston Aug 26 '18 at 0:29
  • @HarryJohnston This question – and especially the answer by Slytherincess that is linked to in it – is a good place to start. The limits and characteristics of the Fidelius Charm are never outright in an inconsistent manner as such (unless you count the content on the old J.K. Rowling site), but what the various little descriptions we have here and there entail end up being contradictory. – Janus Bahs Jacquet Aug 26 '18 at 8:00
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    @Janus, personally, I don't see any of that as a problem. YMMV, I guess. – Harry Johnston Aug 26 '18 at 8:08
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We don't know. The books don't go into any great detail about the exact manner or circumstances in which the Fidelius charm, or any other magic that wasn't absolutely central to the plot, might work. That shouldn't be surprising, really; Harry Potter is a work of fiction, not a magical textbook, and would probably have been far less successful as the former if it read like the latter. :-)

That said, the "how to open doors" example does not seem plausible. The limits of Harry Potter magic might not always be clear-cut, but there are limits.

In particular, note that when the Weasley family had to go into hiding (after Ron was caught with Harry and Hermione) they were first evacuated to Aunt Muriel's and then protected by the Fidelius charm; this implies that the Fidelius charm would not in itself be adequate to protect the Burrow, possibly because the Burrow was too well known a location.

More speculatively, it seems reasonable to suppose that the number of people that already know the knowledge to be hidden would directly affect the power of the Fidelius charm that would be needed to do so; it might also depend on whether those people are friendly or hostile to the intent of the caster, whether or not they are wizards and how powerful their own magic is, how important the knowledge is to them, and perhaps on other factors such as the nature of the knowledge in question, how easy it would otherwise be to rediscover, and so on.

The more powerful and skilled the wizard casting the charm, the more the charm would be able to hide, but there would always be a limit, and even the most powerful wizards would surely be unable to hide knowledge that was as straightforward and universal as how to open a door.

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