Blue Shell

The spiny shell, commonly known as the blue shell, is a power-up in the Mario Kart series that allows a racer to shoot a projectile to whoever is in first place. A character in first place is likely to be hit by a blue shell at some point and once fired, the blue shell is almost always unavoidable. Because of this, the power-up has received a very mixed reception.

History
The power-up first appeared in Mario Kart 64, in the manual of which it was referred to as the "spiny shell." Red spinies date back to the original Super Mario Bros., in which they were among the hardest enemies to defeat. In Super Mario World, Koopas with blue shells move faster than other Koopas and when a Yoshi swallows a blue shell in this game, it sprouts wings and becomes able to fly. Blue shells in Mario Kart are associated with speed, power, and security.

In Mario Kart: Double Dash!!, the shell became more common to characters closer to the lead, whereas in Mario Kart 64, only the 8th (last) place could obtain it. The shell also became unable to hit characters on its way to the lead, as it flies over the track rather than gliding on it. These changed mechanics have become common in Mario Kart games since.

Evasion
It is possible in some Mario Kart games to evade the blue shell despite being in first place. In many games, this can be done by using a boost item at exactly the right time, causing the blue shell to land behind the player rather than on them. However, this is particularly hard to pull off.

In Mario Kart 8, the "Super Horn" was introduced as a way to destroy a blue shell when it comes near your character. This is a rare item in the game, but it was also much anticipated by players.

Reception
The blue shell is infamous as a random element with harsh consequences. A user at At the Buzzer rated the blue shell second in her list of least favourite Mario Kart items, after the "Blooper", stating that the item is absolutely useless when your character is in last place and only exists to "sabotage" the first place racer. Being hit by a blue shell shortly before reaching the finish line is often cited to be incredibly frustrating.

The blue shell is implemented as a "catch-up" system, intended to give the lower-ranking characters a better chance to catch up with the first place and to keep the characters in the race closer together. When a player is uncontested in their lead in Mario Kart for a long time, the gameplay can become dull.

Nintendo Power called using a boost item to dodge a blue shell the "defining moment" of Mario Kart Wii. A miniature replica of the blue shell was added to the limited edition version of Mario Kart 8. The blue shell is also an item in Super Smash Bros. for Nintendo 3DS and Wii U, where it is considered one of the better new additions.