[MUSIC PLAYING] If it sometimes feels like the whole world is pressing in on you, well, that's because it is.

A one-square-meter column of air above your head stretching from here to the edge of Earth's atmosphere contains almost 10 tons of air.

That's a lot to carry on your shoulders.

But if it ever feels like too much, you know what's a great way to get away from it all?

Balloons!

That's not what I meant, guys.

Not like that.

Like this.

[HARP MUSIC] Balloons are magical.

Birds and planes stay aloft by using power to keep air moving over their wings.

We fill blimps and party balloons [HIGH VOICE] with lighter gases like helium, but balloons like those weigh thousands of pounds, and they're not filled with helium.

They're filled with the very same air they're floating in, yet they still go up.

Now that you mention it, why does the atmosphere itself stay up?

That column of air above your head we talked about earlier, well, it's the same mass as a couple of elephants.

But just like elephants, air molecules obey gravity, and I don't see any floating pachyderms around.

Think about it: air is mostly empty space.

If I was an oxygen molecule, I'd have to travel the length of a football field on average before I ran into another one.

But occasionally, they do collide.

Air molecules will fall until the collisions bouncing them up-- the pressure, is balanced by the weight pushing down on them.

Up high in the atmosphere, there's not much air on top of us and not much colliding going on, so the pressure is pretty low.

But down a little lower, we have a big mass of air above squishing down, so things get more crowded and the pressure is higher.

This sets up a gradient all the way down to earth.

The lower we go, the air's more crowded.

It takes more collisions to balance out all that squishing.

Even though it's mostly empty space, air can still hold up a plane.

I can sit in a chair in the sky riding on almost nothing and not fall to the ground.

That is crazy.

You may have heard that the Wright brothers are responsible for humans taking flight, but that's wrong.

120 years before them, in 1783, the Montgolfier brothers had the brilliant and kind of dangerous idea to light a fire underneath a cloth balloon, and humankind took to the skies for the first time.

[MUSIC PLAYING] Heat is what is keeping this balloon up.

You probably heard your whole life that hot air rises, but have you ever really stopped to ask yourself why?

I mean-- well, you know what?

We're going to need a smaller balloon to figure this one out.

Ah.

That's better.

This is the ideal gas law, and it tells us pretty much everything we need to know about how gases behave.

Pressure is basically the force over a given area on a surface.

Whether a balloon inflates depends on the difference in pressure inside and out.

As we heat up air in a balloon, the molecules gain kinetic energy and start colliding with more force on the inside surface.

The pressure goes up.

As the balloon expands, that force is spread across a larger surface area, which lowers the inner pressure, until the pressure on both sides is the same, or until it pops.

But this balloon isn't made out of rubber.

It's pretty rigid, and once it fills up, it can't really get bigger.

But it does have one thing that the rubber balloon doesn't: a hole in the bottom.

Do you know what the mass of air in the balloon envelope is?

This is 105,000 cubic feet.

We can calculate that with a little bit of math.

That's the mass.

It's 105,000-- I don't know.

JOE: Well, I calculated it.

When we first filled up this balloon on the ground, it held about 3,700 kilograms of air.

Add in the basket, the fuel, the people, and of course the balloon itself, and that's another 500 kilograms or so.

A lot of mass to lift, but then we added some-- DEMONIC VOICE: Fire!

So as we're adding heat to this balloon, those air molecules are moving around faster and faster with more energy, and they're pushing on the balloon harder and harder.

But the volume has remained the same.

It hasn't gotten any bigger or smaller.

The pressure's pretty much the same too.

The balloon's not collapsing or popping.

So that only leaves us with one other option.

There must be less air in that balloon than when we were down on the ground.

Hot air has leaked out the bottom.

We're still being pulled down by gravity, but the equal volume of cold air around us weighs more, and it's falling down underneath us.

Balloons don't defy gravity.

They float because of gravity.

Eureka!

This is the principle of buoyancy that Archimedes noticed when he saw his bathtub rise.

It's the reason that ships float, because the weight they hold is less than the weight they displace, and the water falls down around them.

We know that buoyancy is due to gravity because bubbles rise here on earth and they don't rise on the space station.

And you said that flying in a balloon was like what?

Flying in a hot air balloon is the closest thing to being a cloud that I can think of.

We're just floating with the wind, as gentle as can be.

JOE: Flying in a hot air balloon kind of reminds me that we have something in common with fish, that we live in a fluid.

It's just that ours is made of air instead of water.

It's kind of like floating on a bubble.

That's something that can lift all our spirits.

Stay curious.