And the reason for that is this following idea. Now, the volume of the inside of the ball is much greater than the volume that is taken up by these molecules. In other words, any two molecules at any given time are very far apart. 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.