It’s late at night, time when students should be in bed. Anyone who happens to be in the common room at this hour will notice a strange scratching noise. Scratch, scraaaaatch…

Where is that noise coming from? A quick look around will show the common room deserted. Gradually, the scratching becomes louder and more insistent, and soon the scratching is accompanied by a loud screech, like a cat wailing for food. Screeeeech!