mirrors
Never break a mirror. Broken mirrors bring seven years bad luck. That's common knowledge for most.
But do they know why?
Do they know what happens when the mirror fractures, splintering into hundreds of tiny shards, each sharp enough to draw blood? The noise of something shattering causes an instinctual response to turn and find out what broke. All instincts must come from somewhere.
Do they know what they released?
Some say that a mirror is like a portal to an alternate reality: just like ours, but, of course, reversed. They say that we would be able to cross over to this reality if it weren't for the entity in our way: our reflections. The reflections block our paths, convincing us to stay in our world by making us believe there is no other. Look, the baby made a friend in the mirror! they say, or Silly dog, that's just you! Why are you barking so much?
Have they ever stopped to wonder if that person in the mirror is really them?
No one ever actually sees their own face. It's impossible. They can only see through images or reflections, never with only their eyes. Always through something else.
What if we're not what we think we are?
What if that "seven years' bad luck" is them keeping us out more viciously?
What if instead of letting something out, we let something worse in?