Koopa shells are a type of transportation in game.
Green Koopa Shells: These green shells (that look like turtle shells) can be obtained by ! Blocks, Koopas, (but you have to step on them, since if you punch them as Mario, Wario, and Luigi, they will not fall and drop the shell) sea shells, and Orange Brick Blocks. If you walk toward these as any character, you can ride them almost anywhere (even on lava and water and some places that are hard to reach) and you can defeat some enemies in the way. But some obstacles can make you fall, and if you do, the Koopa Shell will disappear. If you want to get off without hitting anything, you can press the R button. Yoshi can also eat them, and if you spit it out, it will go without you.