Remember you can get unlimited amount of 1-ups in Super Mario Bros.? You can do that in Super Mario 3D World! Go to World 5-3, take a Koopa shell to this place, position yourself to the wall, throw it, and quickly tap the jump button. If done correctly, the character will hop on the shell, returns it to the wall, it bounces back, he hops on it, rinse and repeat. It's gonna rain 1-UPs!