We set an object as dynamic when we want that object to fall under the rules set by the physics engine or if we need access to the functionality provided by the dynamics class. The physics engine is designed to mimic physical laws in the game world so that, hopefully, we don't have to pay as much attention to specifying how objects interact. Once you specify an object as dynamic, you can then point it in a direction, apply a force and then see what happens. The object will be subject to friction, gravity, collisions, and so on, unless you specify otherwise. Similar to sensors, we must define the boundaries of the dynamic objects and they appear as green objects in the Scene Viewer window.