World is the core class that enables you to interact with the simulator in an easy and modular way. It handles many time-related events such as adding callbacks, stepping physics, resetting the scene, adding tasks (this will be covered later in Adding a Manipulator Robot), etc. A world contains an instance of a Scene. The Scene class manages simulation assets of interest in the USD Stage. It provides an easy API to add, manipulate, inspect, and reset different USD assets in the stage.