First, gases always move in a straight line. We don't really have anything to demonstrate this with because gravity always pulls objects down. So imagine a bullet fired from a gun, and that bullet goes on at a constant speed in a perfectly straight line. That would be like a gas molecule. Second, gases are so small they occupy no volume on their own. As a group they do, blow up any balloon and you can see how that volume changes. But single gases have no volume compared to other forms of matter. Rather than calculating such a small amount of matter, we just call it zero for simplicity. Third, if gas molecules collide, and they do — remember, these are assumptions — their energy remains constant. An easy way to demonstrate this is by dropping a soccer ball with a tennis ball balanced on top. Because the soccer ball is bigger, it has more potential energy, and the energy from the larger ball is transferred to the smaller tennis ball and it flies away when that energy is transferred. The total energy stays the same. Gases work the same way. If they collide, smaller particles will speed up, larger particles will slow down. The total energy is constant. Fourth, gases do not attract one another, and they don't like to touch. But remember rule three. In reality, they do collide. Finally, gases have energy that is proportional to the temperature. The higher the temperature, the higher the energy the gases have.