Shell (flashing)

A flashing Shell, also described as a flashing kamikaze Koopa shell , is a type of Koopa Shell found only in Super Mario World and its GBA remake, Super Mario Advance 2: Super Mario World. It is created when a yellow Koopa without a Shell climbs into an empty one of any color, including its own after being ejected from it. It becomes nearly invincible in its Flashing Shell, thus becoming a flashing Shell. After its transformation, the Shell attempts to hit Mario or Luigi by spinning around in its shell and dashing towards them. It becomes impossible to separate the Koopa from the shell.

Jumping on a Shell causes the Mario Bros. to bounce upwards, but does not damage the shell. They can only be defeated with a Yoshi, a Spin Jump or a Super Star. If any Yoshi eats one, it obtains all three shell abilities (causing sand clouds, flying, and breathing fire), as long as the shell stays in Yoshi's mouth.

After beating the Special Zone (or completing all 96 exits in Super Mario Advance 2), all Koopa Troopa and Koopa Paratroopa shells become Mario masks. Flashing Shells thus become spinning, flashing Mario masks.