system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a master key open so many unique locks?
gpt
In each lock, there are a series of "pins" that are pushed up by the ridges in the key. If all of the pins are pushed up the correct amount, then the key can be turned and the door unlocked. 

Doors that are designed to work with master keys have a slightly different design. They have the same pins as a "normal" door, but they also have a pin in between the ones that are accessed by the regular key which the master key fits into. These extra pins are the same in all of the locks that the master key works with, while all the regular pins are different. As a result, individual keys only open there respective locks, while the master key can open any of them.