The Green Shell (ミドリのこうら,midori no koura?) is a throwing item (a green shelled turtle-also called "Green Turtle") 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. Unlike Mr. Saturn, the Green Shell doesn't break shields.
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.
Hit or throw a Green Shell, and it'll follow the contours of the environment, causing heavy damager to any character it hits. These Koopa shells have many uses in the Mario series: they've been picked up, kicked, thrown, 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.
Super Mario Kart
Mario Kart 64
In Super Smash Bros. for Wii U / 3DS
The green shell of the Koopa Troopas. The launching power of this weapon is incredible in the right hands, but you should be careful; if it hits a wall, it may bounce back toward whoever threw it. If you see it coming back at you, just jump on it to stop it. It's like playing Super Mario Bros.!
On SSB, if you go on any stage in Training Mode and choose Mario and any character, select 5 Green Shells and have Mario stand in between the shells and the CPU. If Mario executes his Down+B attack, Mario will freeze and hit all the shells at the same time, causing the CPU to go to 999% damage. Mario will continue to do this until the shells disappear. One Fireball attack and launch the CPU far and on small uppercut (Up+A) will launch the CPU instantly.
If a Green Shell is bounced off of Fox or Falco's reflector, it will cause the shell to radically increase the damage and knockback it can inflict. In SSB, once a Green Shell is bounced off of Fox's reflector twice, it becomes powerful enough to one-hit KO an opponent.
Out of the three Shell items, the Green Shell is the only one to appear in every title; the Red Shell hasn't appeared as an item since Brawl, and the Spiny Shell didn't debut as an item until Smash 4.