A lock includes a housing and a pivot is located in the insertion hole of the housing and has the first end thereof connected with a slide. A spring is received in the insertion hole, biases the second end of the pivot. When the slide is positioned at a close position, the slide is applied by the resilient force of the spring as to cover the installation recess so as to prevent dust and water from entering the core. When the slide is positioned at an open position, the slide is applied by a force to overcome the resilient force of the spring, and at least one portion of the slide does not cover the installation recess for convenience of unlocking the lock or replacement of the core. The housing includes a positioning member which positions the cover at open and close positions of the slide.