Yoo Who Cannon is a Special Attack that appears in Mario & Luigi: Bowser's Inside Story. Its name is a pun on the interjection "Yoohoo!" and the word "Who?".
When using this attack, Mario and Luigi will briefly appear in eight barrels 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 attack immediately fails and it will do less damage. It costs seven SP to use this attack. After teaching Mario how to use Blue Shells, the Blue Paratroopas will give Mario and Luigi the 10 Attack Pieces required to use this move. The move is called Yoo Who Cannon because Mario and Luigi say "Yoo Who!" when the attack starts. It, Green Shell and Snack Basket are the only Attack Pieces that are required for the game.
It seems possible that this attack was derived from the Barrel Cannons from the Donkey Kong universe.
Names in Other Languages
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