How do we know how old something is?

For people, we'd ask to see their birth certificate.

For trees, we'd count the rings.

But how do we know how old a fossil is?

Fossils have their own internal clock.

Scientists can read it by looking at the ratio of two different types of carbon atoms.

Of course, every living thing is made of carbon.

Plants grab carbon dioxide from the atmosphere

and use it to form complex organic molecules.

Animals get their carbon by eating these plants.

But there's more than one form of carbon.

Most carbon atoms have six protons and six neutrons.

We call this Carbon 12.

High up in the atmosphere, sometimes cosmic rays hit nitrogen atoms.

This creates carbon with six protons and eight neutrons.

We call this Carbon 14.

Carbon 12 and Carbon 14 behave alike.

But Carbon 14 has one unique and important attribute: it's unstable.

So once an animal dies, the Carbon 14 in its body will start to go away.

Every 5,730 years on average,

about half of the Carbon 14 atoms will decay into nitrogen.

This is its at half-life.

After one half-life, the animal will have

about half the amount of Carbon 14 it started with.

After another half-life, it will have about a quarter.

And after another half-life, it will have about an eighth.

By contrast, the amount of Carbon 12 it has in its body will stay the same.

By measuring the ratio of Carbon 14 to Carbon 12,

we can measure how many thousands of years have passed since the animal died.

Carbon dating works for fossils up to about 60,000 years old.

For older fossils, scientists use unstable elements that have much longer half-lives.

For Scientific American's Instant Egghead, I'm Michael Moyer.