Yoo Who Cannon
From the Super Mario Wiki, the Mario encyclopedia
Yoo Who Cannon is a Special Attack that appears in Mario & Luigi: Bowser's Inside Story and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. A part of its name is a pun on the interjection "yoohoo" and the word "who". It costs seven SP to use this attack in the original game, and fourteen/sixteen BP to use the attack in the remake.
When using this attack, Mario and Luigi will briefly appear in eight Barrel Cannons on screen; the player has to remember where they are. After a barrel rises, the player has to press the or button, depending on which brother was sitting there but, the player has to press that button before the fuse reaches the barrel to damage the enemy. If the wrong button is pressed or if the player takes too long, the other barrels will be destroyed, and the move will prematurely end with little damage being dealt. After teaching Mario how to use Blue Shells, the Koopa Paratroopa Trio will give him and Luigi an Attack Piece Block which houses the 10 Attack Pieces required to use this move. In the remake, Yoo Who Cannons now aim at the enemy to hit upon being used.
This move is available for use in the Challenge Node. It becomes trickier when remembering the order becomes shorter.
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