The Green Shell (ミドリのこうら, Midori no Koura) is a throwing item that keeps sliding in one direction unless it hits a wall or comes in contact with another hitbox, which then causes it to bounce in the opposite direction. When thrown, the Green Shell will continue to slide when it comes in contact with the ground. A stationary Green Shell will also start sliding if it is jumped on or comes in contact with a hitbox. If a sliding Green Shell is jumped on, it will bounce and cease all movement.
The Green Shell's power when sliding on the ground is dependent upon the speed it is traveling at. When it is traveling slowly, the damage and knockback dealt is minimal. When it is traveling fast, the damage and knockback is high. When smash thrown, the Green Shell is very powerful, being capable of KOing under 100%. The Green Shell has a noticeably faster falling speed than other throwable items when thrown, which causes it to often hit opponents with the sliding hitbox instead of the stronger throwing hitbox.
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 kicks the shell, the shell slides across the ground killing whatever is in its path (which could include the player). It would also slide across the ground if the player jumps on the shell. When the shell hits the side of a wall or pipe, it ricochets and slides 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 now 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.
In the Super Smash Bros. series, the empty Koopa shell behaves the same as it did in the games mentioned before. A player can pick up and throw empty Koopa shells to slide across the stage to do damage to enemies, and the shells also ricochet off of walls. Also, the most common shell color for a Koopa is green. 
 Damage Output
 Shell hopping
In Brawl, many characters with high falling speeds can gain a vertical boost in midair by jumping on their thrown shell, but still keeping it. The technique can be repeated infinitely or until the player accidentally drops the shell. It is currently confirmed to work on characters with falling/fastfalling speeds of 2.1 or greater (Squirtle and Marth or higher). It is performed by fastfalling, down throwing a Green Shell, catching, and repeating. A simple way to perform the technique is by pressing Down, Grab button, Grab button, and repeating. Pressing the grab button a third time will result in the character throwing the shell downwards upon their ascent, resulting in losing the shell. The catch must be done quickly after throwing, and the shell can also be caught with a quick aerial attack. As the falling speed decreases, the window for catching decreases as well, meaning that slower fallers like Marth and Squirtle will require extremely precise timing. Characters can also move horizontally after gaining height.
This is most notably performed by Falco, who gains a large vertical distance by shell hopping. Other characters, such as Ganondorf, gain no additional vertical distance if repeated. Fox can catch with his forward aerial to gain additional height. He is also the only character able to shell hop by simply holding down on the Control Stick the entire duration of the technique; all other characters must fastfall again after gaining vertical boost. Shell hopping can be used to perform stage jumps such as the Hyrule Jump.
 Green Koopa Shell
The Green Koopa Shell is a special item obtainable in the Adventure Mode of Super Smash Bros. Melee and in the The Subspace Emissary of Super Smash Bros. Brawl. It can only be obtained by defeating a Koopa Troopa or Koopa Paratroopa. It is slightly different than a normal Green Shell in appearance. It behaves identically to a Green Shell, but when left alone for long enough, it will turn back into a Koopa Troopa. It also does more damage and knockback than a normal Green Shell.
 Description from the Super Smash Bros. instruction booklet
When attacked or thrown, this shell slides around the battle stage, damaging whomever it hits.
 Description from the Super Smash Bros. Brawl instruction booklet
Slides along and launches foes.
 Melee trophy
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.
 Brawl trophy
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.
 See also
 External Links