Simulations have always been important to engineers who use preliminary models and test rigs to find out whether the designs they have thought up will actually work. With the passage of time, computer systems have been developed which are capable of creating telepresence not just in a single virtual world but in a large number of virtual worlds. Such systems are often termed virtual reality (VR) systems. A VR system is a computer people can no longer see. There are two sorts: systems which completely surround people and offer a complete virtual environment, and systems which only offer a window onto a virtual reality. Computer-aided design is a way of designing in which the computer is integrated into the design process. This confers a number of important benefits. The aim of artificial intelligence is to make a computer achieve results that would be regarded as intelligent if they had been produced by a human.