A gas exerts pressure on its container because the molecules of the gas collide with the walls of the container. As gas molecules move randomly and collide with each other and with the walls of the container, they exert a force on the walls. This force per unit area is defined as pressure. Therefore, the pressure exerted by a gas is the result of the constant collisions of the gas molecules with the walls of the container. The other options given are not correct explanations for why a gas exerts pressure on its container.