Rigor mortis is a natural process that occurs in a deceased body where the muscles stiffen and become rigid. It typically begins around 2-6 hours after death and can last up to 48 hours. The stiffness is caused by a lack of ATP (adenosine triphosphate, a source of energy) in the muscles, which prevents them from relaxing. Rigor mortis is one of the key stages of decomposition in the body and can help forensic investigators determine the time of death.
