From the Super Mario Wiki
A Ledge Hammer is an obstacle found in the game Super Mario Galaxy 2. They resemble giant black-and-orange hammers. They appear only in the Clockwork Ruins Galaxy during the "The Ledge Hammer Trap" mission; a total of six appear during the mission, and block the player's path to the star. Ledge Hammers can crush Mario or Luigi under their weight, defeating them instantly. Ledge Hammers also move very quickly, but the hammers can be slowed via the time switch nearby.
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