An apparatus configured to be head-worn by a user, includes: a transparent screen configured to allow the user to see therethrough; a sensor system configured to sense a characteristic of a physical object in an environment in which the user is located; and a processing unit coupled to the sensor system, the processing unit configured to: cause the screen to display a user-controllable object, and cause the screen to display an image of a feature that is resulted from a virtual interaction between the user-controllable object and the physical object, so that the feature will appear to be a part of the physical object in the environment or appear to be emanating from the physical object.