The first step to understanding how VR works is to consider what constitutes the entire VR system. It is tempting to think of it as being merely the hardware components, such as computers, headsets, and controllers. This would be woefully incomplete. As shown in Figure 2.1, it is equally important to account for the organism, which in this chapter will exclusively refer to a human user. The hardware produces stimuli that override the senses of the user. In the Sword of Damocles from Section 1.3 (Figure 1.29(b)), recall that tracking was needed to adjust the stimulus based on human motions. The VR hardware accomplishes this by using its own sensors, thereby tracking motions of the user. Head tracking is the most important, but tracking also may include button presses, controller movements, eye movements, or the movements of any other body parts. Finally, it is also important to consider the surrounding physical world as part of the VR system. In spite of stimulation provided by the VR hardware, the user will always have other senses that respond to stimuli from the real world. She also has the ability to change her environment through body motions. The VR hardware might also track objects other than the user, especially if interaction with them is part of the VR experience. Through a robotic interface, the VR hardware might also change the real world. One example is teleoperation of a robot through a VR interface.