The unknown. In many of these stories, there are things like unseen hands or strange noises from an unknown source. It plays on our fear of the things we can't explain or see clearly. It makes our imagination run wild and think of all the worst - case scenarios. For instance, when you hear a scratching sound on the wall but don't know what's causing it. It could be a small animal, but in the context of horror, it could be something much more sinister.
The sense of isolation is also a very scary element. When the character in the story is alone, whether in a big empty house or a dark forest. It heightens the fear because there's no one to turn to for help. And any danger seems much more threatening without the possibility of immediate assistance. For example, a person alone in a cabin in the woods and hearing strange footsteps outside.
The suddenness is really scary. For example, in stories where a normal situation suddenly turns terrifying like a quiet room and then a loud, unexplained noise. It gives no time to prepare.
One: I woke up in the middle of the night to a dripping sound. When I turned on the light, I saw a figure hanging from the ceiling. Two: I heard a knock on the door. I opened it and there was no one, but then I felt cold hands on my neck.
The sudden appearance of unknown entities. For example, a figure suddenly showing up in a supposedly empty room.
The sudden and unexpected events. For example, a character thinking they are alone and then suddenly seeing a shadowy figure right behind them.
The suddenness. For example, like 'The phone rang in the dead of night. It was a voice from the grave.' The sudden phone ring in the middle of the night is already scary, and then it's a voice from the grave which is very spooky.
The scariest elements often include the unknown. For example, in stories where there is something sensed but not seen clearly. Like 'I felt a presence in the dark corner. When I turned on the light, the presence was gone but a cold draft remained.' Here, the unknown presence is very frightening.
Well, the one where 'I woke up in a coffin. As I screamed, I realized I was buried six feet under.' is quite terrifying. Just the thought of being trapped alive underground is a nightmare.
The sense of helplessness is a major scary element. Like when a character realizes that there's no way to escape from El Coco once it has arrived. It gives a feeling of doom and despair. For instance, in a story where a person is trapped in a small space and El Coco is approaching, they know they can't get away. This powerlessness is what makes these stories truly frightening.
Another really scary one could be 'My phone rang at 3 am. The caller ID said it was me.' It plays on the idea of the self being something other or unknown, and calls at that time are always spooky.
The abnormal events like a clock ticking backwards. It defies the normal order of things and creates a sense of unease. And the shadowy forms that are often left unexplained. Our minds tend to fill in the blanks with the scariest possibilities when something is not clearly defined, which is what makes these elements so scary in the '10 scary 2 sentence stories'.
I think 'I heard a child crying in the basement. But I live alone.' is quite terrifying. The idea of a crying child in a place where no child should be, especially when you live alone, plays on our fears of the unknown and the supernatural. It makes you wonder what could be down there, and the thought that it's not a normal, explainable situation is very frightening.