Blue Shell
Blue Shells are blue Koopa Troopa shells that first appeared in Super Mario World. This was one of the only games they were worn by Koopa Troopas. The Koopa Troopas wearing Blue Shells were just like Koopa Troopas wearing Red Shells. They were a bit more intelligent than normal green-shelled Koopa Troopas. They did not fall off cliffs and simply turned around when reaching a ledge. They also walked faster. They are a rare type of shell, even 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 give Yoshis wings that allow them to fly until it swallows the shell or spits it back out, in which case it will behave like a green shell does, flying out at high speed and knocking over all enemies in its way. In New Super Mario Bros., Mario or Luigi can wear Blue Shells, which give them the ability to perform a shell spin move similar to that of a Koopa Shell which has been kicked or hide inside the shell, where they cannot be damaged. Finally, like the Frog Suit, they move faster in water. In multiplayer mode, there was a single Koopa Troopa wearing one of these in the ice level.
In the Mario Kart Series, they are deadly weapons. They are similar to Red Shells, except these home in on the driver currently in first place. In Mario Kart games made after Mario Kart Super Circuit, Spiked Shells are replaced with the Flying Spiked 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. These varieties were spiked and called Spiked Shells.
The flying versions of these were called Winged Spiny Shell. 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. Winged Spiny Shell are much more efficient than the previously replaced Spiked Shells. So far, these have only appeared in Mario Kart: Double Dash!! and Mario Kart DS.
Super Mario Strikers had Blue Shells as items. If the Blue Shell hit anyone, the character who got hit would freeze for some time.
New Super Mario Bros. had a Blue Shell not as a weapon but a Power-up.
