From the Super Mario Wiki
Stackin' Stus are a variant of Strollin' Stus in Super Mario Sunshine. Ten particularly-small Strollin' Stus are stacked on a Giant Stu. Stackin' Stus appear exclusively in Pinna Park, in episodes 1, 3, 5, 6, 7, and 8 underneath a wooden bridge near the roller coaster. Mario can use F.L.U.D.D. to spray and knock down the Strollin' Stus until the Giant Stu remains. He can then jump on it to defeat the Stackin' Stu for a Blue Coin. Defeating the Giant Stu defeats all Stackin' Stus that made up the stack. They bear resemblance to the Goomba Tower form of Goombas, which debuted nine years later in Super Mario 3D Land.
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