Or, in simple words: the scientific theory. We're capable of a thought process that no other species in the LDC is capable of despite numerous dumb-down explanations we've given about it. They haven't worked it out yet regardless.
This is really screwy to think about. Learning IRL is pretty domain-general. There isn't much difference between learning that it's good to talk to this person because she's been nice before and learning that it's good to mix these substances because they've always exploded nicely before. To have a conversation with someone perfectly intelligent and have them completely fail to grasp a simple concept,no a set of concepts, over and over again must be mind-boggling. I suppose the fairies feel the same about magic.
Human Soldier: Looks like... five demons under that extremely precarious, huge, sharp rock.
Elf Soldier: They haven't seen us yet. We have plenty of time. What were you saying?
Human: So, Marissa was supposed to feed my cat yesterday-
Elf: Haha, don't tell me; she forgot again.
Human: Right. So Mittens was pissed, and so was I. I'm done trusting her to other people.
Elf: That's why I don't have pets. I could watch her, though, if you need it.
Human: Thanks. I appreciate it, really. Anyway, we could probably just poke that rock and kill all the baddies. What do you say?
Elf: ...
Human: ...
Elf: Sounds complicated.
Human: (How do you dress yourself in the morning?)
Does this apply to angels, too? Gabriel's comment on her relative ignorance due to the circumstances of her birth implies a sort of workaround, and so does the tech level of Heaven. At least for human-based angels, and probably nightmares, too.
Ooh!
Idea: make an elf play a war game where he has to achieve some objective (kill the bad guys) with resources (soldiers). Then... swap all the names around, so he thinks he's gathering up children with a bunch of adults or something. What would happen?