Recent experiments and their analysis suggests a common mechanism for collision-induced processes from inelastic to reactive collisions.  The driving force is the exchange of momentum and is the constant mechanism in a wide range of events.  Energy conservation provides the boundaries within which the mechanism may operate and the variation of these from system to system gives great richness to the observed outcomes.  The operation of the mechanism and the boundary conditions may be depicted in diagramatic form based on relationships that emphasise the reality of molecular shapes and sizes. In some cases, the boundary conditions force population to focus into very narrow distributions, a situation that can survive many collisions without degradation.