Mirrors. For a layer of glass coated with a thin lining of silver, polished metal or just a still pool, they have a disproportionately large level of mystique and hold upon the human mind. I guess it's because they show you what you cannot see- however hard you try, without a mirror you can never see yourself.
But the world isn't the same behind the mirror. It's all the wrong way round. Right becomes left and left becomes right, the virtual image within unable to perfectly replicate reality.
But what if they didn't try to? What if that wasn't a reflection? It's a common motif in fiction. This thin, silent surface is actually a barrier to another world, one fundamentally different but also the same. If the you in the mirror has the same mind, the same consciousness as the one looking in, then why shouldn't they mimic your every movement?
And that flash, in the corner of your eye- can we see the invisible spirits and demons of their world, and can they see ours?
There was a time when men treated calm pools with respect, reserving them for divination, and left offerings by them for whatever they happened to name their little people- fey folk, djinn, daimons. But such times as long past and they will not come again.