It is said that a gas fills all the space available to it. Why doesn’t the atmosphere go off into space?

Question

What competing factors determine the thickness of the atmosphere? What causes atmospheric pressure? Give an example of something that we could do in class that would demonstrate that the atmosphere has pressure.

Answer 1

some air does escape to space,but for the most part gravity is holding it near the planet.  The simplest example i can think of it to create a vacuum in some sort of tube, place the tube in a liquid and the liquid gets pushed up into the tube part of the way by the air pressure. The basic idea is the vacuum is removing the air pressure from above the surface of the liquid and not able to push back down.

Answer 2

For the same reason you don't fly off into space! Gravity!