Sometimes they want to pass on information about their life, death or things that were more important than life was to them. There are guardian spirits, who may have suffered a tragedy in the past and warn others in similar circumstances. Some spirits stay on for no apparent reason. They cause things to move from place to place, make noise and generally upset the routine of their surroundings. Poltergeists are usually associated with children before or during puberty.
Poltergeists usually fade as puberty ends.
Most ghosts seem to fade away over time. They are most energetic after whatever event creates them. As time goes by their manifestations become less frequent. At first some ghosts are seen, heard and felt. As time goes by they may fade away, and may only be heard.