The Earth is 4.565 billion years old, give or take a couple million years or so.

How do scientists know that?

There's no "Established in..." plaque stuck on a cliff somewhere.

No, geologists got there thanks to a handful of radioactive elements.

With radiometric dating, scientists can put an age on dinosaurs, and yes, even good old Mother Earth.

So how DO you date a rock?

Well, it takes some good chemistry.

Lots of people have tried to date the earth.

Archbishop James Ussher famously used the Bible and a host of other historical documents to settle on the evening of October 22, 4004 BC.

He was off by just a little bit.

Things started to get more scientific during the Industrial Revolution.

Geologists looking at layered rocks realized they were formed from gradual deposition of sand, silt and other sediment -- and that must have taken a long, long time.

Once scientists came around to the idea, they tried some new approaches to deciding its age.

Lord Kelvin had an idea to date the Earth by figuring out how much it had cooled down since it was formed.

He settled on somewhere between 24 and 40 million years old.

Better than Ussher, but Kelvin still didn't come close.

One reason why is he didn't know that there was something inside the Earth that was actually keeping it warm: radioactive rocks and minerals.

The discovery of radioactivity in 1896 paved the way for radiometric dating, which lets scientists put an accurate age on just about anything.

Even the planet.

To understand how, let's take a closer look at what happens to radioactive elements as they decay.

We start with an atom of a radioactive element.

Scientists call that the parent atom.

When it breaks down, it releases, well, radiation.

That transforms it into a whole different element, which scientists call the daughter atom.

When you have a whole bunch of radioactive atoms together, the overall rate of decay from parent to daughter is constant.

When half the parents have become daughters, that's called the half-life of that element.

That half-life is the key to radiometric dating.

By counting up the number of parent atoms of a given element and the number of daughter atoms around it, scientists can figure out how long those atoms have been there.

"There" could be a dinosaur bone, a big ol' trilobite or some other rocky, really old thing.

No not him.

That's better.

To get an accurate age for something as old as the Earth, geochemists have to be a little picky about which elements they go out looking for.

The parent and the daughter element have to be stable enough that they'll both still be around after billions of years -- otherwise there'd be nothing to compare.

Uranium is a popular choice for dating rocks.

It's a radioactive element that's sometimes incorporated into crystals of zircon, a mineral formed as magma cools.

Cubic Zirconia also provide Cheapos with a "cope-out" engagement ring option.

Uranium decays through a series of radioactive elements into a daughter atom, lead.

Zircon doesn't incorporate lead while it's forming.

That's important because it helps geochemists know that lead they find in zircon comes from decaying uranium.

So when they collect a rock in the field, bring it back to the lab and count the atoms in the zircon crystals, the ratio of lead atoms to uranium atoms in any zircon crystal tells geochemists when that crystal formed.

But here's the thing: geologists have never found pieces of Earth that have been around since the planet was born.

Our world is an active place.

Rocks are constantly being smashed and melted and reformed, so it's no wonder very little has lasted all that time.

The oldest zircon we've found is pretty close: somewhere around 4.4 billion years.

So why do geochemists say the Earth is even older?

Scientists arrived at the 4.565 billion years by expanding their dating pool.

They used the same radiometric techniques to date meteorites.

These space travellers are practically unchanged since the very beginning of our solar system, when they, the Earth and other planets formed.

Since they don't have to deal with magma and weather and other geology business, the rocks on meteorites are a snapshot of the solar system's early days.

Geologists LOVE dating meteorites.

You might even call them rock stars.

They're like time capsules that crash into the earth.

The minerals in them can be radiometrically dated just like any others.

And that, my friends, is how you date a rock.

In less than a hundred years scientists went from numbering the Earth's age in thousands of years to billions, all thanks to a handful of elements and the scientists who revealed their secrets.

Not that they're satisfied, of course.

Geochemists are still fine-tuning their estimates of the age of the Earth and looking for more evidence to support or diminish their theories.

But really, the only Rock I have my eyes on right now is Dwayne "The Rock" Johnson.

That guys dreamy.