Players are given a large degree of control over the appearance of the world through the Edit Town menu. Each world has outer boundaries beyond which Sims cannot travel, and Sims cannot visit other worlds (except 'sub-worlds') without NRaas Traveler, a mod which enables this functionality. Major buildings and landmarks in each world, including locations where adults can work and where children and teens can attend school, are represented by rabbit holes, which can be interacted with but cannot be looked into by players. Worlds are open for Sims to explore, and Sims can be controlled even when not on a lot. The Sims 3 introduced the concept of an expanded 'open world.' Especially early in the development of the game, these were called towns, as they were much larger and more complex than the neighborhoods of the previous games.