One of the great things about "Flat Earth" theory is that it provides some great opportunities for thinking about, investigating, and explaining a variety of basic science concepts.
Now, everybody knows that if you have an alien in your spaceship, the best course of action is to blow a hole in a window, and the alien will get sucked out. Problem solved!
So we all know that if you've got some air under normal pressure, and there's a vacuum next to it, then the air will rush over to fill the vacuum.
But the Earth's atmosphere is under normal pressure, and there's nothing between the atmosphere and the vacuum of space. So why doesn't all the air on Earth get sucking into space.
The answer of course is gravity. Gravity is holding the atmosphere down. But how does that work exactly? How does gravity keep the air in my room at a constant pressure?
Well to understand how that, you've got to think about what the atmosphere actually is.
The atmosphere, the air, is a gas, a mixture mostly of nitrogen and oxygen. Like all gasses it's just a bunch of molecules flying around.
Now we are used to thinking of the atmosphere in terms of air pressure, but to understand that, you have to consider the motion of individual molecules. Let's do some thought experiments in a simplified universe.
Let's say the universe contained nothing at all except for some gas molecules. Since molecules are really just like tiny little objects they behave like other objects would in space. The gas molecules would just move in straight lines, not changing direction unless they happened to collide with each other. The universe would just continue like this, gas molecules moving around. (Eventually they would cluster together and form nebulae and stars, but that's another story.)
So suppose we add a planet into the mix? How would that change things?
Suddenly our universe has a strong gravity well. All the nearby gas molecules are attracted down to the planet, so they tend to move in that direction. Now adding a planet like that has created a whole bunch of potential energy, so things speed up and slosh around for a bit (see video, above). But eventually they settle down.
What do we find? The planet has acquired an atmosphere! No only that, but it's one where the density of the atmosphere decreases as you get higher. Most of the molecules near the planet's surface are bouncing around, colliding with each other, and colliding with the objects around them. It's those collision that make air pressure.
But the ones off in space are just following ballistic trajectories, and because there's so few molecules there's far fewer collisions, and so very little pressure, if any.
So we can see that there's no reason for the atmosphere to fly off into space. All of the atmosphere is being pulled down towards the planet. The only reason why air rushes to fill a vacuum down on earth is because the molecules are bouncing around so much they just fill up any empty space.
Let's look at this in a bit more detail. Here the bottom of the screen is surface of the planet.
Again we see a pressure gradient, with a vacuum at the top and sea level at the bottom. You can also see there's no compelling reason for the molecules at the top of the atmosphere to whizz off into space. Some of them do, but gravity keeps most of them returning to earth.
Another thing you can see here, there isn't really an edge to the atmosphere, it just gets thinner and thinner, and the molecules get further and further apart. It never really ends though. Even in the space between galaxies, the hardest vacuum there is, there's still about one atom per cubic meter.