VR systems: O consolidate mixed-media training, automates the selection and administration of courses, assembles and delivers learning content, and measures learning effectiveness. O are a new way to design and deliver online learning where learners can collaborate with each other, watch short videos, and participate in threaded discussion groups. O let workers model an object on a computer and print it out with plastic, metal, or composite materials. O sometimes require the user to don special clothing, headgear, or equipment. O enable acquiring, storing, and disseminating knowledge documents in a virtual world