It gets in your head! Seriously, Knock-knock terrifies on a mental level. Most of the time, games are terrifying because the creatures are just freaky, and they're trying to kill you. But in Knock-knock, there's no gore, no blood, no dismemberment, and the ghosts aren't that terrifying. Design wise. But there's just something about them, and the way the whole game comes together, that makes the hair on your neck stand on end.