Blue Shell

Blue Shells are blue Koopa Troopa shells that first appeared in Super Mario World. This was one of the only games where they were actually worn by Koopa Troopas. The Koopa Troopas wearing Blue Shells were smarter than Koopa Troopas wearing Red Shells. They were also a lot more intelligent than normal green-shelled Koopa Troopas. They simply turn around when reaching a ledge. They also walked faster, and they were the only ones who could kick their own shells. They are a rare type of shell, rarer than Red Shells.

Like other Koopa Troopa shells, the Blue Shell will be launched when the Koopa Troopa wearing it is jumped on. It can also be held and thrown. There are also flying Red Shells which appear in Super Mario RPG: Legend of the Seven Stars. They are not to be confused with Parabuzzy or Buzzy shells, which are darker and more spherical. During the events of Super Mario World, Blue Shells, when eaten, give Yoshi wings that allow it to fly until it swallows the shell or spits it back out, in which case the Blue Shell will behave like any other shell, flying out at high speed and knocking over all enemies in its way.

In Mario Smash Football, as well as in Mario Strikers Charged Football, Blue Shells appeared as items. They are usually collected in a pack of three. When used, they will be fired at the opposing players, however, they do not home in, like the Red Shell does. If the Blue Shell happens to hit a player, they will instantly freeze in one spot. This will render them unable to move, and, thus, unable to get the ball. The effects wear off after a few seconds. Unfortunately, these shells can freeze both the users teammates, or the user him/herself. If the shells fail to hit anyone, they will continue bouncing around the stage until they either hit a player, or just wear out on their own. A more effective version of this item, is the Giant Blue Shell.

In New Super Mario Bros. there is a Blue Shell item. It is not used as a weapon, but as a power-up. The shell turns Mario or Luigi into Shell Mario or Shell Luigi respectively. When ducking, Shell Mario/Luigi is invulnerable to almost anything. Also, they can Shell Dash. Blue-shelled Koopa Troopas that leave Blue Shell power-ups when stomped appear in multiplayer. They respawn periodically.

A Blue Shell (taking its appearance from Mario Kart: Double Dash), referred to as Spiny Shell, appeared in Super Smash Bros. Brawl as a sticker. If used in the Subspace Emissary, the sticker would increase the launch power of a specific character by eleven.

The Blue Shell is back in Mario & Luigi: Bowser's Inside Story, as an item found in overworlds. If Mario and Luigi are outside Bowser's belly, Mario can go in Blue Shells (and turn into Shell Mario from New Super Mario Bros.) by striking Blue Shell blocks. Luigi can then hit Mario using his hammer, and this will make Mario go into the shell, spinning straight forward. This can be used to hit blocks in areas which Mario and Luigi cant directly go into. It behaves similarly to Kooper and Koops's abilities in the Paper Mario series.

In Mario Kart
In the Mario Kart series, they are deadly weapons. In Mario Kart 64 and Mario Kart Super Circuit they are similar to Red Shells, except they home in on the driver currently in first place, destroying anything in its path in the process. The non-flying variety of these were called Blue Spiny Shells. If the player in first place used it, they would instantly be crushed by their own weapon, which is why it's best to use it when the player is NOT in first place. In Mario Kart games made after Mario Kart Super Circuit, Blue Spiny Shells are replaced with the Flying Spiny Shells, most likely because of their low reliability in Mario Kart 64 and Mario Kart Super Circuit. They could be missed and fired at a wall, but would otherwise never fail in hitting the target. They would sometimes crash into a wall, if fired near a curve and were unable to make certain long jumps. In Mario Hoops 3 on 3, this non-flying version of Blue Shell appears in the item list of the game. After homing in on a player, it will explode, affecting anyone within the blast radius.

The flying versions of these were simply called Spiny Shells. After firing one, it flies over the course, homing on the current contestant in first place. After the shell reaches its destination, it will hover for a brief moment over its target, then smash into the ground, giving off a massive shock wave causing the target and anyone near him to bounce around out of control. Spiny Shells are much more efficient than the previously replaced Blue Spiny Shells. These shells have appeared in Mario Kart: Double Dash!!, Mario Kart DS and Mario Kart Wii. In Mario & Sonic at the Olympic Games for DS, the Spiny Shell appears as an item in Dream Canoe.

Trivia

 * The only non-enemy characters to wear a Blue Shell are Kooper, Parakarry, and Koopley.