I heard about this on NPR. A Reddit thread dedicated to two-sentence horror stories. I thought "That's cool, but..."
The guest on the radio then read her personal favorite, and I was convinced (as it became mine, too, when I read the entries at this link):
I begin tucking him into bed and he tells me, “Daddy check for monsters under my bed.” I look underneath for his amusement and see him, another him, under the bed, staring back at me quivering and whispering, “Daddy there’s somebody on my bed.”
-Juan J Ruiz
So what do you think? What can you do to scare a reader in just two sentences?
Note: My advice is to not read too many of the Reddit stories. That'll dim your spark pretty quickly.