From the Super Mario Wiki
Bowser's Shell is a Special Item exclusive to Mario Kart: Double Dash!! that Bowser and Bowser Jr. can receive. Much like any other Special Item, King Boo and Petey Piranha can also receive it. It is a gigantic shell that, when released, bounces around the track like a normal Green Shell, toppling any kart that it hits as well as knocking down the rear character, making the racers drop their items and temporarily disabling them from switching.
After Bowser's Shell hits the walls of a course roughly fifteen times, the shell breaks apart. Unlike many other items, the Bowser Shell can resist being destroyed by some other items, and even bounces off another special item should it collide with one.
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