Developing elastic applications should be easy. This paper takes a step toward the goal of generalizing elasticity by observing that a broadly deployed class of software - the network middlebox - is particularly well suited to dynamic scale. Middleboxes tend to achieve a clean separation between a small amount of per-flow network state and a large amount of complex application logic. The authors present a state-centric, systems-level abstraction for elastic middleboxes called Split/Merge. A virtual middlebox that has appropriately classified its state (e.g., per-flow state) can be dynamically scaled out (or in) by a Split/Merge system, but remains ignorant of the number of replicas in the system.