You know the old saying: That like people, no two snowflakes are exactly alike.

But is that really true?

Their intricate beauty is so delicate... so fleeting.

It's made them one of nature's great mysteries.

But that incredible complexity actually comes from very simple building blocks.

High up in the clouds, snowflakes begin as water vapor.

Water molecules whiz around, bouncing off of each other.

When the temperature cools, the molecules slow down and start sticking to one another.

They form a hexagon: six-point symmetry, the core structure of a snowflake.

As each one grows, it builds on that basic geometry, creating a crystal lattice.

That's why you tend to find snowflakes with six arms.

Not five, not seven.

From there, the variety just explodes.

The shapes are practically infinite.

So what are the chances of finding two snow crystals that look exactly the same?

In his lab at Caltech, Physics professor Ken Libbrecht has figured out how to solve that mystery.

Inside a chamber, he makes snowflakes from scratch...

He starts with humid air, and drops the temperature until ice crystals start to form.

When they get heavy enough they fall.

He catches them on a chilled plate, where he can watch them grow using time-lapse photography.

As he boosts the humidity, arms begin to extend out from the corners.

If he cools the air even more, branches shoot off the arms.

Libbrecht can create a bunch of crystals on the same plate.

Like these two- they're growing in exactly the same conditions.

And look!

Twins!

But these perfect copies only exist here... in the lab.

Outside up in the clouds it's unpredictable.

The temperature, humidity and air pressure are constantly changing.

Each snowflake takes its own path as it falls to earth.

And that solo flight means they all grow a little... differently.

So each one really is unique, shaped by its own individual journey through the world.

Hey guys, it's Lauren.

We've got a few more tiny mysteries for you.

Find out how roly polies ventured out from the ocean to conquer your backyard.

Or how just one handful of sand can tell you the history of the entire planet.

And while you're at it, share us!

See you next time.