Consider a sky lantern with a candle. The candle's heat increases the internal energy of the air molecules inside the lantern, which causes the expansion of air and inflates the lantern. Due to the expansion, work is done by the air molecules on the lantern. The air density inside the inflated lantern is reduced, which creates lift. In this example, the change in internal energy of the system is equal to the net heat transfer into the system minus the net work done by the system. This equation is a generalized form of energy conservation known as the first law of thermodynamics. Conventionally, Q is positive or negative, depending on whether heat is added to the system or removed from the system. Also, W is positive when work is done by the system and negative when work is done on the system. For any thermodynamic process, the change in internal energy is path independent and depends only on the system's initial and final equilibrium states. Thus, the internal energy is a state function.