Poltergeists (from the German, "noisy ghosts") usually manifest as loud sounds, strange electrical effects, and the unexplained movement of objects. At one time, these phenomena were thought to be due to ghosts, but after decades of investigations by researchers, notably by William G. Roll, the evidence now suggests that poltergeist effects are actually caused by the living. The MMI itself may be produced by one or more individuals, often (but not always) troubled adolescents. The term "RSPK," meaning "Recurrent Spontaneous PK," was coined to describe this concept. The events themselves--like all forms of spontaneous MMI--are intrinsically meaningful and can be understood much in the same way as a dream metaphor to explain the underlying psychological issues that triggered the phenomena. Investigations are therefore performed in the same manner as would a newspaper reporter, looking at the who, what, when, where, and why. Some parapsychologists believe that poltergeist activity are a way of relieving stress through the physical expression of unconscious feelings.