And the increase in the number of molecules increases the system's kinetic energy, collective kinetic energy. And that, in turn, pushes on the walls of the balloon. And this push exerts a pressure or A force on the outside molecules, the surrounding molecules in the air. And this does work on those molecules, expanding the balloon and moving those molecules away. Okay? So collectively, the molecules or the atoms within the system, within a stationary system expand by doing work on the surrounding molecules.