Should a homogeneous mass change properties in a consistent manner, the time for one half of the mass to change is the half life. Determining that the mass change is unfluctuating is important when many half lives will occur. Example, bacteria on a food source will multiply steadily with accurate half lives within the constraint food is continually supplied. Best example of half life is radioactive decay. Short of a nuclear reaction, an unstable isotope will decay undeviating from a constant. 100 radioactive atoms "A" will become 50 "A" and 50 "B" in one half life.