How does a master key work?

The super of our apartment building is always going into everyone's apartment when something needs to be fixed. There must be a hundred apartments, but he only carries one key around with him. How does he get into all those apartments?

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How does a master key work?

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The Master Key Workbook: A Complete Method of Self-Mastery and Goal Attainment Based on The Master Key System, the Legendary Book by Charles F. Haanel: Anthony R. Michalski, Anthony R. Michalski: 9780976111115

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