Well, one possible 'scary two - line story' could be 'I heard a knock on the door at midnight. When I opened it, no one was there but I felt a cold hand on my shoulder.' Another could be 'The old house was empty. As I walked in, I saw a shadow that moved on its own.' And a third one might be 'She walked through the graveyard. Suddenly, a whisper called her name from an unmarked grave.'
They are effective by creating immediate tension. For example, 'The phone rang at midnight. A raspy voice said 'I'm coming'.",
"Answer2":": 'Scary two line stories' work well because they rely on the reader's imagination. In a story like 'The moon was full. A howl that wasn't from a dog echoed.', the reader has to fill in the blanks about what might be making that howl. It's this sense of mystery that makes them effective.
There might be a story of a cursed object. A girl found an old locket in an antique store. After she put it on, she started having nightmares of a shadowy figure. Everywhere she went, she felt a cold presence. She finally threw the locket into a river, and the nightmares stopped.