Shell Mario and Shell Luigi are the respective Mario Bros. in a shell. The only appearance of Shell Mario and Shell Luigi is New Super Mario Bros. To hide in the shell, the bro must dash for a certain amount of time. This power can kill most enemies, not including Whomps. If a bro uses the shell on Bowser Jr., he will be stunned for a little bit of time. While inside the shell and while running, Mario is invincible to most enemies and objects, main weakness being pits, lava or bosses. If Shell Mario/Luigi falls into water while spinning, the start swimming. If Mario collects any other power ups, he instantly loses the shell ability. Super Mario 3D World references this by allowing you to go into shells for a brief amount of time.