Simulations are used for many purposes, because in a simulation things can be controlled that might not be easy to control in real life. They can use smaller versions of an object or system for testing or practice. Smaller versions of things, like space ships or rooms, can simulate the real thing. They can also be safer, in place of a thing that is dangerous, like when astronauts practice in a swimming pool rather than in outer space. Astronauts practiced in simulations of the moon before they went and landed there. They use simulations of spaceships that move the way real spaceships do. Simulations often use computers. Computers are used to make simulations of what a spaceship's route might be on the way to the moon, to help plan the mission.