A video game with nonlinear gameplay presents players with challenges that can be completed in a number of different sequences. Each player may take on (or even encounter) only some of the challenges possible, and the same challenges may be played in a different order. Conversely, a video game with linear gameplay will confront a player with a fixed sequence of challenges: every player faces every challenge and has to overcome them in the same order. A nonlinear game will allow greater player freedom than a linear game. For example, a nonlinear game may permit multiple sequences to finish the game, a choice between paths to victory, different types of victory, or optional side-quests and subplots. Some games feature both linear and nonlinear elements, and some games offer a sandbox mode that allows players to explore an open world game environment independently from the game's main objectives, if any objectives are provided at all. A game that is significantly nonlinear is sometimes described as being open-ended or a sandbox, and is characterized by allowing players to measure progress through self-determined goals, independent of scripted game elements. Level design A game level or world can be linear, nonlinear or interactive. In a linear game, there is only one path that the player must take through the level, however, in games with nonlinear gameplay, players might have to revisit locations or choose from multiple paths to finish the level. As with other game elements, linear level design is not absolute. While a nonlinear level can give the freedom to explore or backtrack, there can be a sequence of challenges that a player must solve to complete the level. If a player must confront the challenges in a fixed order nonlinear games will often give multiple approaches to achieve said objectives. A more linear game requires a player to finish levels in a fixed sequence to win. The ability to skip, repeat, or choose between levels makes this type of game less linear. Super Mario Bros. is an early example of this, where the player had access to warp zones that skipped many levels of the game. In some games, levels can change between linear design and free roaming depending on the objective of the stage. Super Mario 64 is an example where the main stages are free roam, while the levels where Bowser is encountered follow a straight path to the end.