For the more you know about how dead bodies decay--the biological and chemical phases they go through, how long each phase lasts, how the environment affects these phases--the better equipped you are to figure out when any given body died: in other words, the day and even the approximate time of day it was murdered. The police are pretty good at pinpointing approximate time of death in recently dispatched bodies. The potassium level of the gel inside the eyes is helpful during the first twenty-four hours, as is algor mortis--the cooling of a dead body; barring temperature extremes, corpses lose about 1.5 degrees Fahrenheit per hour until they reach the temperature of the air around them. (Rigor mortis is more variable: It starts a few hours after death, usually in the head and neck, and continues, moving on down the body, finishing up and disappearing anywhere from ten to forty-eight hours after death.)