When one object exerts a force on a second object, the second object MUST exert a force back on the first object. An interaction occurs when two objects (or two systems of objects) exert forces on each other. As a result, an interaction is always composed of a PAIR of forces, and we call the two forces in an interaction an action force and a reaction force. So, each interaction is made up of an action and a reaction. In other words, a SINGLE interaction is made up of an action force and a reaction force.