Jessi: Sometimes on a nice clear day, we can see something special in the sky...a hot air balloon! Have you ever seen one? Have you ever wondered how a hot air balloon works? How does it get up there? And how does it stay up there? Well, it’s all about the air!

There’s nothing special inside the balloon that makes it float. It’s just the same air that we’re breathing right now! So how does that keep the balloon up there?

Well, even though you can’t see it, air isn’t nothing--it’s something. Air is made of matter! Anything that you can touch, or feel, or that takes up space, is made of matter. So air is made of matter, just like water is made of matter, and so are the clothes you’re wearing, and the food you eat -- even your body is made of matter! So what kind of stuff is matter made of?

It’s made of tiny things called molecules, that are way too small to see, but they’re there! So both inside and outside the balloon, there are many molecules of air, moving around and taking up space. But you know what’s fun about molecules? When they heat up, they start to move differently! That’s right! When something gets warm, the molecules that it’s made of start to move around faster, bouncing around and spreading out. And that’s the important part: They spread out.

So let’s think about how that would happen in a hot air balloon. When the balloon is on the ground and ready to go, it has some air inside it. And that air is the same temperature as the air outside of the balloon.

So the air molecules are all moving around at the same speed, and are just as far apart from each other, both inside and outside of the balloon. But then, the balloon pilot makes the air inside the hot air balloon... hot! You see, these balloons don’t have engines like airplanes do. They have big heaters that send flames up into the balloon!! So when the pilot turns on the flames, it heats up the air inside it! That makes the molecules of air start bouncing around inside, and they start to get further apart from each other.

As these molecules spread out, they start to fill up the balloon even more! And when those molecules of air inside the balloon start to take up more space, we say that the air becomes less dense. Soon, the air inside the balloon becomes less dense than the air outside.

Now, maybe you remember when we talked about why some things float, and other things sink. We learned that things that are less dense than water will float to the top, while things that are more dense will sink. And the same is true with our balloon! The hot air inside the balloon is less dense than the cool air outside -- so the balloon can float up! In fact, warm air always floats up.

Have you ever noticed that in the winter, when you have a heater on, the downstairs of your house is usually a lot cooler than the upstairs? That’s because the warm air in your house is rising to the top! So, in a hot air balloon, the hot, less-dense air inside the balloon rises up on top of the cooler, denser air outside. And that makes the balloon rise!

Now, how does the balloon get back down? Well, the pilot opens a flap at the top of the balloon that lets the hot air out! As the balloon fills with cooler, denser air, it begins to sink gently back down to the ground.

It may be hard to believe that something so big can be carried by little molecules we can’t even see. But that’s what makes hot air balloons -- and science! -- so amazing!

Do you have a question about anything else that floats or flies? Do you want to know how something works? Then ask a grownup for help, and leave us a comment below, or send an email to kids@theSciShow.com! We’ll see you next time!