Well, it comes from the observation that gases are easily compressed and mixed very well. And this is because the distance between the molecules is much larger than the size of the molecules. Now let's look at our inflated ball. Within our ball, we have lots of different molecules. But the difference between any two molecules is much greater than the size of the molecule itself. And that's why we can compress it because when we compress it, there's lots of space for the molecules to move.