In the Mario Kart series, a Green Shell will shoot forward in a straight line. They do not home in on an opponent. Unlike the Red Shell, the Green Shell is able to ricochet off of walls. After about five times it bounces off walls, the Green Shell will break. In Mario Kart 64, Green Shells remain on the track until they either fall off or hit a racer or item. Green Shells come in singles or triples. In the latter case they will circle around the kart until all three are fired. In this state, they can be used as a barrier to block contact with other shells (except Blue Shells) and Bananas. Also, if the driver comes in contact with any other person, they will be knocked out by the shells. In Mario Kart: Double Dash!!, Triple Green Shells were only available with Koopa Troopa and Paratroopa. Their version was held in the kart rather than forming a barrier.
Green Shells are actually green Koopa shells.