The theory that explains the behavior of gases is the kinetic molecular theory. This theory posits that gases consist of tiny particles in constant, random motion. The energy and speed of these particles are directly related to the temperature of the gas. The theory also suggests that gas particles collide elastically with each other and the walls of their container, leading to variations in pressure and volume. Overall, this theory provides a framework for understanding various gas laws, such as Boyle's law and Charles's law, that describe gas behavior under different conditions.