The Green Shell (ミドリのこうら, Green Shell) is a throwing item that has appeared in all four of the Super Smash Bros. games. Based on the item in the Super Mario series, the Green Shell has a variety of unusual properties when thrown.
As an item
Upon being thrown, the Green Shell will deal significant damage and knockback to opponents; however, it has a higher specific gravity than most other items, and as a result, it will fall onto the ground more quickly than other items. Unlike other items, the Green Shell will continue to move forward when it hits the ground, and it will continue to deal damage and knockback as it slides. The resulting hitboxes, however, are not as powerful as the throwing hitbox, and the Green Shell's output becomes weaker and weaker as it begins to slow down. Furthermore, opponents can immediately stop the trajectory of the Green Shell by merely jumping on it, then use it against the original wielder; as the Shell also ricochets off walls and other barriers, it is also possible for a player to hit by their own Green Shell.
If the Green Shell is not picked up by players, it will stay in place until it is hit by a damaging hitbox, after which it will begin moving with a speed directly proportional to the knockback of the hitbox. Jumping on it will also cause it to begin moving either left or right, but with relatively little movement.
In Single-Player modes
In Melee's Adventure Mode, Brawl's Subspace Emissary, and Smash 4's Smash Run, Koopa Troopas and Koopa Paratroopas can appear as regular enemies. Upon having sufficient damage being dealt to them, they will retreat into their shells, and they can then be used as a standard Green Shell item. The resulting Shell is actually more powerful in terms of damage and knockback than the regular item, but not using the item after a time period can cause the Koopa to reemerge from the shell.
Unusually, the Green Shells found from actual Koopa Troopas are smaller than the actual item.
Description from the Super Smash Bros. Brawl instruction booklet
Slides along and launches foes.
Hit or throw a Green Shell, and it'll follow the contours of the environment, causing heavy damage to any characters it hits. These Koopa shells have many uses in the Mario series: they've been picked up, kicked, swallowed and even spit out. Mario just uses them as tools to pummel Bowser's numberless henchmen.
An item that, when thrown or jumped on, careens across stages and hurts any player it hits. Green shells follow the environment's contours as they go and reverse direction when they hit walls. They really send opponents flying, so they're very effective as throwing weapons. If a shell's approaching, stop it by jumping on it. This technique is used in the Mario series.
Smash 4 trophy
In Super Mario Bros., there were turtle-like enemies called Koopas. When the player jumped on a Koopa, it would hide in its shell for a short period of time. When the player kicked the shell, the shell would slide across the ground, killing whatever is in its path, including the player. It would also slide across the ground if the player jumps on the shell. When the shell hit the side of a wall or pipe, it ricocheted off and slid in the opposite direction. Koopa shells were also an item that could be pulled out of the ground and thrown at enemies in Super Mario Bros. 2. In Super Mario Bros. 3, the player could also pick up a Koopa shell and throw it at enemies. In Super Mario World, if the player jumps on a Koopa, rather than hiding in its shell, it instead gets knocked out of the shell completely; leaving the shell empty. There were also instances in Super Mario World where a player can just find an empty shell.  The shells had also became a major item in the Mario Kart series and the Green Shell serves as the logo for the Shell Cup.
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