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A Boo Block (originally known as Boo Buddy Block, also known as Block Boos in Yoshi's Story) is a combination of a Boo and a Block first appearing in Super Mario World. When looked at, it seems to be a normal, gray block. But if Mario takes his eyes off it, it slowly begins to transform into a Boo. They are actually required to clear a certain Ghost House, Choco-Ghost House, as Mario must lure them towards him, turn around, and jump on and leap off them to reach a high ledge, where the exit door awaits. In Super Mario World: Mario to Yoshi no Bōken Land, when the Boos turn into blocks, they still show their face, but in the original game, their faces disappear.