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Part 4522 is a LEGO piece in the shape of a hammer. It is meant to be connected with Part 983, the LEGO hand. A hammer is a tool meant to deliver blows to an object. It consists of a stick with a heavy metal on top. The essential part of a hammer is the head, a compact solid mass that is able to deliver the blows to the intended target without itself deforming.
A new hammer is included with the Thor minifigure.