What is the source of the energy that does the work in an adiabatic expansion of a gas?

Explanation

From the first law of the thermodynamics, `U_(f)-U_(i) = Q-W`,i.e.,change of internal energy = heat gained - Work done.
For an adiabatic expansion, `Q = 0 . So, U_(f)-U_(i) = -W or, W = U_(i)-U_(f)`, i.e., Work done = decrease in internal energy. This means that internal energy is covered into external work in an adiabatic expansion and the temperature of the gas decreases.