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from the Real Life
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Room States are what determines what is in a room at what time in a Metroid game. Most rooms have only one room state, but many have two, three, or even four room states. Different room states are triggered by certain events occurring within the game. This can range from collecting an item to defeating a boss.
Room States determine which areas are accessible at what times, and change throughout the game. Some of the changes are aesthetic, such as the Acid Rain ceasing to fall in the Landing Site of Crateria after the acquisition of Power Bombs. Others are more significant, such as the damage that Sector 5 suffers after Nightmare breaks free from captivity. Changes in Room States are generally irreversible, but are usually for the better, with one exception being Chozo Ghosts and Dark Pirate Commandos appearing throughout Tallon IV and Aether. This is because they can be a nuisance to the player when they are exploring.