And that's because whenever I have this container and does this container out of air, if I open this up, what will happen to gas molecules? Well, remember, gas molecules are moving at high speeds. So if I open something up, they will escape. And also, because they're moving at high speeds, they don't feel intermocular attractions, so there's nothing stopping them from exiting into the container, so they can completely expand the container. And that means, on a similar note, different gases will mix completely because there is no inter molecular interactions between any two gas molecules. Now, compare this to liquids and solids, right?