Green Shell

The Green Shell is a throwing item that appears in Super Smash Flash and Super Smash Flash 2. It is one of the 4 items from the shell family, which also includes the Red Shell, the Spiny Shell, and the Yellow Shell. It keeps moving in one direction unless it hits a wall and then bounces in the opposite direction. It is an item from the Mario universe.

Origin
In Super Mario Bros., there were turtle-like enemies called Koopas. When the player jumped on a Koopa, it would hide in its shell for a short period of time. When the player kicks the shell, the shell slides across the ground killing whatever is in its path (which could include the player). It would also slide across the ground if the player jumps on the shell. When the shell hits the side of a wall or pipe, it ricochets and slides in the opposite direction. Koopa shells were also an item that could be pulled out of the ground and thrown at enemies in Super Mario Bros. 2. In Super Mario Bros. 3, the player could now pick up a Koopa shell and throw it at enemies. In Super Mario World, if the player jumps on a Koopa, rather than hiding in its shell, it instead gets knocked out of the shell completely; leaving the shell empty. There were also instances in Super Mario World where a player can just find an empty shell. In the Super Smash Bros. series, the empty Koopa shell behaves the same as it did in the games mentioned before. A player can pick up and throw empty Koopa shells to slide across the stage to do damage to enemies, and the shells also ricochet off of walls. Also, the most common shell color for a Koopa is green.

Following its appearances in the Super Smash Bros. series, this shell behaves similarly in the Super Smash Flashseries of fan games but was a bit limited in the first Super smash Flash and coming fully functionable in the reboot, Super Smash Flash 2.

In Super Smash Flash
In Super Smash Flash, Green Shells, referred to as simply as "Shell" due to Red Shell's absence, appear commonly and can be thrown, however, it will keep moving and will ignore any wall of object in front of it and will simply disappear once it reaches the blast line, it cannot be stopped nor reflected.

In Super Smash Flash 2
In Super Smash Flash 2, it appears as a throwing item that can be used by both picking up the Shell and throwing it, or by jumping on it. It hurts all players, including the one who used it. It deals 11% of damage and has a moderate knockback.

Trivia

 * Koopa Troopas with Green Shells walk off the edges of cliffs; that's why the Green Shell falls off the edge of stages.