Serial killers appear to be sexually dysfunctional and have a low self-esteem. Their methodical rampages are often a part of an elaborate fantasy that builds to a climax at the moment of their murderous outburst. Serial killers generally murder strangers with cooling off periods between each crime. Many enjoy cannibalism, necrophilia and keep trophy-like body parts as mementos of their work. Serial killers are sadistic in nature. Some return to crime scenes or grave sites of their victims to fantasize about their deeds. Many like to insert themselves in the investigation of their crimes and some enjoy taunting authorities with letters or carefully placed pieces of evidence.