Poltergeists Terrible spirits that terrorise people

Poltergeists are a worldwide phenomenon - they are ghosts or demons. Poltergeists usually nest in houses and make themselves known by scratching, knocking noises and cold spots, but they also throw furniture around the rooms. In Europe and the USA, poltergeists are often seen as the souls of dead people who have died violently and suddenly.
What do you think poltergeists are?