Chairs move about by themselves. Walls shake from loud, unexplained banging. Water drips from a ceiling. Hairbrushes disappear for days, only to reappear in their place on the dresser. These are some of the classic symptoms of a poltergeist haunting. From the German for "noisy ghost," a poltergeist refers to phenomena usually credited to mischievous spirits or ghosts and are characterized by psychokinesis or other physical manifestations. Although ghosts may sometimes be involved, most poltergeist incidents are a kind of psychic phenomena, usually centered around a living "agent." Cases have been cited almost since the beginning of recorded history.