Knocking at the door

A brother and sister were home alone one night, and before their parents left them and told them not to open the door no matter what, but they did not pay attention. After a few hours they were in bed, and they heard a knocking. They shrugged it off and tried to return to sleep, but they kept hearing the knocking.

So the sister got up to see who was at the door, while the brother listened from his bed. After a while, he was so scared that he decided to stay with his neighbors. He headed for the front door through the darkness. When he got there, his neighbors were already awake, and happy to see him. They had heard a murderer was spotted a few blocks down. In the morning, they discovered the sister's body and a discarded knife on the brother's bed.