So when I compress this ball, these molecules have lots of room to get closer, right? So this model gets closer, this one gets closer, this one gets closer and eventually they all get closer so I could push them in. Now, if this guy was replaced with liquid or solid, liquid and solid is much more dense and that means I would not be able to push it together because all the molecules are close together. And that leads straight to the third point. Gas molecules exert a force on whatever they hit. And this force can be calculated in terms of pressure.