Don't Blink

"Creatures from another world, only statues when you see them. Lonely assassins, they used to be called. No one quite knows where they came from but they are as old as the universe, or very nearly. They have survived this long because they have the perfect defense system, they are quantum locked. They don't exist when they are being observed. The moment they are seen by any other living creature they freeze into rock. No choice, it's a fact of their biology. In the sight of any living thing they literally turn to stone. And you can't kill a stone. Of course a stone can't kill you either, but then you turn your head away. And you blink. And oh yes it can. That's why they cover their eyes. Their not weeping, they can't risk looking at each other. Their greatest asset is their greatest curse. They can never be seen. Loneliest creatures in the universe. And I'm sorry. I'm very, very sorry. But it's up to you now. Your life could depend on this. Don't blink, don't even blink. Blink and you're dead. They are fast, faster than you can believe. Don't turn your back, don't turn away, and don't blink. Good luck."
-The Doctor, Blink