Well, why is that? Well, the reason for that is because all these molecules are moving at tremendous velocities and every time they hit the container, hit the walls, they exert a force or a pressure. Pressure is simply force per some given area. And that means the added effect of all these molecules hitting the wall will be tremendous. And that's exactly why I can't push this ball any further than, say this, right? So whenever we talk about force of molecules, force of gas molecules, we really want to talk about the pressure.