Decomposing starts almost immediately, with the skin going through several changes in colour as the blood stops circulating leaving the body an ashen color. However different cells die at different rates. For example, brain cells die within a few minutes, whilst skin cells can survive over 24 hours after death. Soon after death rigor mortis occurs, which is caused by a complex chemical reaction (involving lactic acid and myosin), which forms a gel like substance which creates the body's stiffness.