Clearly, ARK interactors behave much like constraints on the physical objects in the alternate reality. Thus, creating and modifying interactors exemplify ARK’s constraint-oriented features. A programmer can generate new interactors by creating networks of message boxes. As a simple example, consider developing a frictional force interactor by creating a message box which adds a force to an object proportional to the negative of its velocity [Smith 1987]. The message box can be set to continuously send its message, and when its behavior has been verified, the programmer can convert it to an interactor.