A Koopa Shell.
Koopa Shells are what Koopas wear as protection. Depending on the game, this can be separated from the Koopa or the Koopa encouraged to withdraw inside, and then picked up and thrown. In 2D Mario platformers, they harm and defeat most enemies while moving along the ground, and bounce off walls and other objects. In 3D Mario platformers, their use varies.
Super Mario World
When Yoshi eats a shell, he gains special abilities.
- Green Shell: No effect.
- Red Shell: When Yoshi spits it out, he breathes fire.
- Blue Shell: Yoshi will gain wings and the ability to fly.
- Yellow Shell: When Yoshi lands with a Yellow Shell in his mouth, he will create a stomp that can defeat enemies on the ground.
The Yoshi's color also has an effect on what ability it obtains when eating a shell. For example, if a Yellow Yoshi has a red shell in its mouth, it would be able to spit fireballs and make sand clouds when stomping on the ground.
New Super Mario Bros.
In this game, there are Green Shells, Red Shells, and Blue Shells. Green and Red Shells are worn by Koopas and can be carried and thrown as projectiles. Blue Shells are power-ups that transform the player into Shell Mario/Luigi.
Super Mario 64/DS
Only Green Shells exist in this game, and to get them, Mario must jump on a Koopa, or get one out of an Item Box. Mario can then ride on it until hitting a wall or other objects. While riding on a shell, Mario can also move up steep slopes with a shell, which wouldn't be possible to traverse otherwise.
Super Mario Galaxy
Only green and red shells exist, and they are thrown in a straight line. They disappear upon coming into contact with anything, and harm enemies. Koopa Shells are also used as a way to propel Mario through water at high speeds.
Mario Kart series
In this series, there are Red, Green, and Spiny Shells, none worn by Koopas, all of them usable items within races. The Green Shell shoots straight forwards and bounces off walls until it hits a player, or if it hits the walls too much. Red shells are homing, but if it hits the wall it will disappear, and if it is thrown backwards it will just travel in a straight line. Spiny shells race around the track and disappear upon hitting the person in first place. In some games, it has wings and flies, and harms only the leading racer and anyone else nearby when it strikes, In others, it makes its way along the center of the track on the ground, and harms any karts it touches along the way.