A lock includes a movable unit and a lock core structure including a restriction unit and a lock core. The restriction unit has a passage penetrating therethrough to form a first opening and a second opening on two opposite sides of the restriction unit. An extension wall is formed in the passage to face the second opening. The lock core is disposed in the passage to couple with the restriction unit and is selectively movable relative to the restriction unit in an axial direction. The lock core has an end face facing the extension wall and an extension portion disposed on one side of the lock core close to the first opening and selectively protruding outside the first opening. The movable unit selectively contacts the lock core and is able to reciprocally rotate, wherein rotation of the movable unit is linked to axial movement of the lock core.