[MUSIC] For thousands of years, humans, and plants and animals long before that, have been using frozen "sky water" to keep warm.

Which sort of doesn't make sense.

Because snow is cool.

You might even say it's... ice cold.

YEEAAHHHHH!

No one knows for sure who built the first igloo, but with the right fit and the right physics, snow can actually warm you better than the inside of a tauntaun.

"You'll be ok, Luke!"

So, how can something cold keep you cozy?

[MUSIC] The vast, frozen Arctic is one of the most forbidding environments on our planet, yet, the Inuit have managed to live there for about 5,000 years.

Out on the pack ice, winter temperatures reach 50 degrees below zero , and when it's that cold, surviving means finding shelter.

It's not an area known for its forests, so nomadic hunters learned to build with the only thing available: snow.

Eskimo languages really do have dozens and dozens of different words for snow, because there are a lot of different types, and the type of snow you choose can dictate whether your igloo keeps you warm, or turns you into a Homo sapiensicle.

To understand this, we need to know a little something about being cold.

When your body temperature starts to plummet - you're feeling heat leave you.

Cold can't move into your body - in fact, there is no such thing as cold.

Where have I heard that before?

Oh, right!

Think of heat as an actual quantity of stuff:.

The more you give away, the colder you feel.

This trading of heat can happen three different ways: by convection, conduction, and radiation.

All three are at play in an igloo.

A person inside will radiate body heat, which moves around the igloo by convection, and is lost through the walls by conduction.

This is exactly what happens in your house.

Living insulation does the same thing.

Fatty tissues like blubber help stop heat transfer in whales and seals, but for animals who don't have as much junk in the trunk, they cover themselves in air.

Sea otter fur, for example, is about a thousand times denser than human hair.

It's snuggly stuff "This is the softest thing I've ever felt in my life.

You are adorable!"

...but the secret to its insulation power is in its texture.

Otter fur is spiky, so it traps insulating air molecules.

And that is exactly what snowflakes do.

Powdery, fresh snow can be up to 95% trapped air.

This makes it an excellent insulator, but the same way you have to pack it in your hands to make a snowball, it isn't dense enough to build with.

Solid ice, on the other hand, makes a good windbreaker, but it's too heavy to lift.

Inuit hunters took the Goldilocks approach: the secret to good igloo snow is somewhere in the middle.

Traditional igloo blocks aren't molded, they're cut out of the ground.

That tightly-packed ground snow is dense enough to hold up, but because it still has far more air pockets than a block of ice, it's light, and still a pretty good insulator.

As usual, animals figured this one out long before humans.

Polar bears, groundhogs, even birds like grouse all make snow burrows to stay warm.

And even before that, plants were tucking into snow to avoid death by freezing.

During the warm months, heat energy from the sun builds up in soil, and just like the the roof above your head, a deep covering of snow prevents that heat from escaping onward and upward.

This snowy blanket above stops ice crystals from forming inside plant roots, and shoots, and seeds.

Not freezing to death is a pretty good motivator for any animal to get crafty, but our big primate brains took it one step further with igloos.

Their engineering maximizes warmth and stability.

Cartoon igloos look like flat-bottomed half-spheres, but in reality, they're neither of those things!

If you were to slice a real igloo in half, you'd see a shape called a catenary.

This gradually sloping shape is the same one that would form if you held a chain from both ends and let it droop.

A catenary arch distributes weight more evenly than a half circle, without bulging or buckling.

In fact it's one of the most stable arches in nature, so sound that we still use it today.

Inside, snow houses are carved in different levels.

The hot air rises, and the cold air sinks down into the lower part, and away from where you would eat, sleep, and chill.

To boot, body heat melts the innermost layer of the walls, strengthening the barrier between you, your airy snow-block insulation, and the frigid great beyond.

When you live in an igloo, you act as a living furnace.

Over time, the temperature in your icy abode can hover some 40-60 degrees above the surrounding air, but bring a friend to your igloo party, and you'll get warmer, faster.

Stay cozy, and stay curious!

"Hey, you remember that thing I said about eskimos having all those different words for