Radiocarbon, or carbon 14, is an isotope of the element carbon that is unstable and weakly radioactive. Carbon 14 is continually being formed in the upper atmosphere by the effect of cosmic ray neutrons on nitrogen 14 atoms.

It is rapidly oxidized in air to form carbon dioxide and enters the global carbon cycle.

This is called the half-life—the amount of time required for one-half of a given number of atoms to disintegrate. The plot of the number of tiles as a function of the number of turns looks like this: Again, I made radioactive spheres disappear when they decayed.

This is fine, because when carbon-14 decays, it produces nitrogen-14. But you could imagine that if you knew that the sample started with 20 percent blue spheres and you knew their half-life, then you could determine the age by examining one frame from the animation.

Some examples of the types of material that radiocarbon can determine the ages of are wood, charcoal, marine and freshwater shell, bone and antler, and peat and organic-bearing sediments.

I know can be hard to wrap your head around, so let's model it with a six-sided die. You can use Lego bricks, pennies, beans—anything you can easily count. Every time you roll a one, put that object into a separate pile.

Count the remaining objects and repeat the process until half of them have decayed. It took a while, but we finally got pretty close to 40 tiles left.

As you learned in the previous page, carbon dating uses the half-life of Carbon-14 to find the approximate age of certain objects that are 40,000 years old or younger.

In the following section we are going to go more in-depth about carbon dating in order to help you get a better understanding of how it works.