Side-Levels are optional levels that are unlocked by playing through the main story, usually through Keys, though LittleBigPlanet 3 introduced Side-Quests, which are unlocked by finding quest-givers in the hub levels. They tend to be shorter and simpler than the main levels. In LittleBigPlanet, Side-Levels did not unlock any Prize Bubbles. Apart from fun, the only incentive to play them was to unlock the "Play" Trophy. LittleBigPlanet 2, however, introduced the Score Sensor, and since then they have regularly offered prizes for getting a certain score.