One of my daughters was just reading Chicken Little to me. I don’t know if you are familiar with Chicken Little, but she is a chicken that runs around telling people “The sky is falling”. In my normal fashion, I started thinking about the plausibility of this. What would fall? What would you look for? Then I figured it out. The sky IS falling. It is ALWAYS falling and it has always been falling.
What is the sky? I am assuming the sky is the air. I will treat the air as a gas of single particles (which it isn’t, but that’s ok). So, why does this sky (air) do what it does? If you look at each individual sky particle, its motion is governed by two things.
1) Gravity. The gravitational force makes each particle fall. Without this, all the air would escape the planet. (this would be bad)
2) Collisions with other particles. This is what prevents the “sky” from collapsing.
I actually talked about this some before [when I talked about MythBusters and the Lead Balloon](http://www.dotphys.net/files/lead_balloon.html). The best way to see this interaction between gravity and collisions for a gas is with the excellent [PHET simulator](http://phet.colorado.edu/new/simulations/sims.php?sim=Balloons_and_Buoyancy)
If you adjust the “gravity” you can see that there are more particles down lower, but that a particle in between collisions looks like a projectile. Thus, I think it is ok to say that the sky IS falling.