Each isotope decays at a particular rate. The amount of time it takes half of any Collision of the isotope with nuclei to Decay is called the half-life. Let's illustrate the concept of Half-Life using some special marbles that behave like radioactive nuclei. suppose a box holds 1024 blue marbles. you close the box and leave. coming back 10 minutes later, the open the box and you find that it now has 512 blue marbles and 512 red marbles. Presumably 1/2 of the 1024 original blue marbles changed into red marbles. Again you still a box and leave. Coming back 10 minutes later, there are 256 blue marbles and 768 red marbles in the Box. Again half of the previous 512 marbles changed into red marbles. In other words half of half or 1/4 of the original blue marbles is still in the box.