An interactive computer presentation or model designed to imitate a real object or environment. Essentially, a simulation is a mathematical description of an object or situation that uses further mathematical computations to evaluate changes to the object or situation. Examples of simulations include flight simulators which use elaborate graphics and sound to train pilots (some are so effective that they even spawn a very real phenomenon known as simulation sickness) or the various means by which eyes have evolved in nature. Virtual reality is also a kind of simulation.