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Cosmetics are a set of different custom skins, accessories, boxes, bags and books that the player can get by a spin. There are two types of spins: skin spins and custom spins. Skin spins can get you custom skins while custom spins can get you custom accessories, boxes, books and bags. Each type of spin has three qualities: starter, golden and dragon. Starter spins can get you wooden, stone, golden and diamond. Golden can get you stone, golden, diamond and amethyst. Dragon spins can get you golden, diamond, amethyst and reidite. Once you get a cosmetic, you have to press the button under the respective cosmetic that you got. Search for it and you can equip it.
Skins can be accessed in the left middle of the game's starting screen after skin spin. They come in various colors and emojis including the original "blue" one, which stayed in the game for the longest amount of time (was from the first release). Accessories can also be optionally added to any of the skins (there are no theme limits).
Accessories are small features which can be optionally added onto your character. They are separate and independent from skins. There are currently 57 of them - 12 unlocked only after reaching level and 45 from skin spin which can be opened at every level. Access to accessories is granted simultaneously with skins, by using skin spin in starve.io. Only one accessory may be used at a time. Like skins, accessories serve no purpose other than to alter your appearance. Accessories were added in the BAKERS ARE YOUR BANKERS update.
Boxes are features that appear either when the player drops an item or dies. The drop item one is called a "loot box". The box when the player dies is called a "Death Box" or "Box of Dead People". Only two custom boxes may be used at a time (one loot box one death box). Like skins, custom boxes serve no purpose other than to alter their appearance. You can get them via the custom spins.
Books are little customizables that you can put in your book. You can get them via the custom spins.