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Radiocarbon then enters animals as they consume the plants (figure 2).

So even we humans are radioactive because of trace amounts of radiocarbon in our bodies.

If we know what fraction of the carbon atoms are radioactive, we can also calculate how many radiocarbon atoms are in the lump.

With time, those sand grains fell to the bottom bowl, so the new number represents the carbon-14 atoms left in the mammoth skull when we found it.

The difference in the number of sand grains represents the number of carbon-14 atoms that have decayed back to nitrogen-14 since the mammoth died. The sand grains in the top bowl fall to the bottom bowl to measure the passage of time.

The standard way of expressing the decay rate is called the half-life.5 It’s defined as the time it takes half a given quantity of a radioactive element to decay.

So if we started with 2 million atoms of carbon-14 in our measured quantity of carbon, then the half-life of radiocarbon will be the time it takes for half, or 1 million, of these atoms to decay.

The radiocarbon half-life or decay rate has been determined at 5,730 years.

Next comes the question of how scientists use this knowledge to date things.

The sand grains that originally filled the top bowl represent the carbon-14 atoms in the living mammoth just before it died.

It’s assumed to be the same number of carbon-14 atoms as in elephants living today.

Half the original quantity of carbon-14 will decay back to the stable element nitrogen-14 after only 5,730 years.

(This 5,730 year period is called the half-life of radiocarbon, figure 5).6 At this decay rate, hardly any carbon-14 atoms will remain after only 57,300 years (or ten half-lives). The decay of radiocarbon follows the exponential decay law, whereby the percentage decrease in the number of parent atoms per unit time is constant.

Although many people think radiocarbon is used to date rocks, it is limited to dating things that contain carbon and were once alive (fossils).

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