Blue Shell

From Mariopedia

The Blue Koopa Shell is an item from the Mario Kart series and a power-up in New Super Mario Bros.; it's effect and appearance is completely different in the two. In the Mario Kart series, the shell has wings (and spikes in some games), but not in New Super Mario Bros.

[edit] Mario Kart

When used, the shell will immediately fly to the racer in first place, hover for a few seconds, and make contact with the racer to make a blue explosion similar to that of a Bob-omb. When used in first place, the Blue Shell will fly around the course and target the person in first place, in this case, the one who activated the shell. The affected racer is flung up into the air, losing a few seconds.

[edit] New Super Mario Bros.

In this game, the Blue Koopa Shell is a collectible power-up with a priority higher than the Fire Flower but lower than the Mega Mushroom. It attaches a a blue shell from a Koopa to the body of Mario or Luigi, and, if they achieve a certain speed, they withdraw into the shell, spinning along the ground, though still able to jump. This method of travel is slightly slower than usual, but makes the player invincible to most enemies.