Okay, so how balloons expand? Balloons expand because you blow in air. In other words, you blow in oxygen molecules, water vapor, you blow in carbon dioxide, you blow in nitrogen molecules, and so on. 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.