# Q & A: Floating in Air?

Q:
What makes things float in air?
- Marcellus (age 9)
Chicago
A:
Marcellus -

The reason that things float in air is because they're lighter than air is. It seems a little bit strange to think that air actually weighs something, but it really does! Air is made up of lots of /really/ little things, like nitrogen and oxygen atoms. These atoms hardly weigh anything, so gravity doesn't pull on them very hard, but they weigh enough to be held near the ground. (If you've ever heard of someone going up very high on a mountain, you've heard that there's not as much air up there. That's because air does weigh something, and gravity pulls it down towards the earth.)

How much something weighs for how much space it takes up is what scientists like to call "density." Something like rocks has a higher density than something like styrofoam. This is because rocks weigh more for how much space they take up than styrofoam does. If you were to mix a bunch of rocks in a box with a bunch of styrofoam peanuts, you'd notice that the rocks would settle to the bottom, and the styrofoam would end up on top. This is because things with higher densities (things that weigh more) get pulled down by gravity harder than things with lower densities (things that weigh less) do.

It works exactly the same way with air. Some things, like helium gas, have lower densities than air does. This means that they weigh less for how much space they take up. So if you have a balloon full of helium, it will float high up, because it has a lower density than air does. Things like rocks, on the other hand, have higher densities than air (they weigh more), so they'll just sit on the ground.

Thanks for the question! Hope this helps!

-Tamara

(published on 10/22/2007)