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A bonus stage (also known as a bonus level or bonus round) is a special level within a video game designed to reward the player or players, and typically allows the player to collect extra points or power-ups. Often a bonus stage will have no enemies or hazards, or may contain them but the player character is invulnerable to attack from them and cannot be harmed. Some games have bonus stages where the player character can be harmed by enemies or hazards, but will not lose health or lives if killed, instead the stage just ends and play continues with the next regular stage. Many bonus stages need to be activated or discovered in some manner, or certain conditions must be satisfied to access them, making them secret levels. Some games also contain bonus stages that are not secret but are compulsory and must be played, often at the end of the regular stages.
In some games, bonus stages have an interface and game paradigm that is completely different and disconnected from the rest of the game, as in the slot machine bonus stage of Super Mario Brothers 2. Other bonus stages use the same gaming paradigm as the rest of the game, as in the car smashing bonus stage of Street Fighter II or the bonus stages in Super Monkey Ball where you collect bananas to earns extra points and lives. In the Bomberman series, they also have enemies in bonus stages with the goal being to defeat as many enemies as you can to earn items and power-ups. In addition, the player won't lose a life from touching the enemy or being caught in a bomb blast during the bonus stage. Many games feature bonus stages somewhere between the two extremes.
The type of modern game which most commonly uses bonus stages is the platform game.
The first games containing a bonus stage are widely believed to be Namco's Rally-X (1980) and Galaga (1981). In the latter, the level would begin with the text "Challenging Stage" and would feature enemies that do not attack. The player is then rewarded extra points based on how many enemies could be eliminated.
Since few modern games use points as an incentive for gameplay, most modern bonus stages reward the player with power-ups rather than points. Modern games have often blurred the traditional distinction between bonus stages and ordinary levels. Most first-person shooter games allow players to enter bonus rooms within larger (non-bonus) levels to temporarily power up. Some games have optional bonus stages which must be discovered and whose completion impacts the mechanics of the rest of the game, as in the "Star Road" levels of Super Mario World. There are bonus levels in most Crash Bandicoot games, save for Crash Twinsanity and Crash of the Titans.