This isn't a duplicate, but I couldn't respond here:
Why is space a vacuum? Also, why doesn't air from the Earth escape into space?
Someone said "space isn't that kind of vacuum" and "it doesn't suck".
If that is so, why not? How can this essentially pure vacuum possibly not be strong enough to overcome gravity and suck off the atmosphere?
Imagine you have two identical rooms with a door between them. One is a vacuum and the other is filled with air. The air molecules are moving in straight lines until they bounce into each other and get deflected.
Now, you open the door between the two rooms. Air molecules near the door that just happen to be traveling towards the door have nothing to bump against. So a lot of them will do so. So over a short period of time, there will be few air molecules near the door. So molecules going into that direction will also be able to keep going in that direction without bumping into anything. This chain reaction keeps happening until there is now about the same number of molecules in the two rooms.
That's what vacuums "sucking" is: molecules that are randomly traveling towards the vacuum having an unimpeded path which frees space for more molecules traveling by random in that direction.
So now look at earth. It's almost the same thing, but you need to add gravity. The molecules aren't going in random directions. Gravity pulls them down and away from the vacuum of space. So instead of many molecules being on their path towards space in the first place, few are. And so our atmosphere doesn't just escape into space.
"Things" go into a vacuum, because of a difference in forces pushing them in, a vacuum does not suck anything into it. But the force from Earth's gravity is pulling things down, so they stay here. Without gravity, the atmosphere would drift away, not be pulled violently into the void.
Let me take the example of the two rooms from the other answer.
By definition, a vacuum contains nothing, absolutely completely nothing and especially nothing that can do something like pulling in molecules. There is nothing there, so no mechanism of any kind by which the vacuum can do anything.
But the other room, that is full of gas, at whatever pressure you like, is obviously different and because the air molecules are present, there is a mechanism by which it can do something.
The air molecules can travel into the empty room because, on average some of them will be heading in that direction anyway. The higher the pressure, means the more of them, just by chance, will head in to the vacuum room.
This process will keep going until both rooms contain the same number of molecules. But the vacuum has done nothing to affect this process, whereas the room with air molecules has done something. The air molecules orginally in it, especially if they are at moving fast ( under pressure) have bounced off each other like mini billiard balls and, just by chance, sent some of them into the vacuum room.
