This game, within any system, may be called 'Banging into Each Other by Pairs'. A few major things are conserved during this game. Flip a coin, for example. You and the coin exchange. The coin goes upward due to the momentum imparted to the coin by your thumb.
Gravity pulls the coin back down in an exchange with the Earth.
When a ball is thrown against a wall, momentum drives it towards the wall, which imparts motion to it in the opposite direction. Neither momentum nor matter is lost. To describe the behavior of a system, then, one looks at how the flow of matter, energy, momentum, and electric charge spread out in this game that the actors within the system are playing. The purpose of the action is to tell the story of the collective. For a group of molecules in a balloon, the story of the collective may be to 'form a gas'.