system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If each lock has a different key how does a master key work?
gpt
A normal pin tumbler lock has two sets of pins, key pins and driver pins.  When you insert the key into a lock cylinder the shape of the key lifts the key pins to their sheer line (the place where the driver and key pin meets). When all the pins are lined up the cylinder can spin.  

Locks that accept master keys have two sets of key pins. One set lines up when the users key is inserted, the other set lines up when the master key is inserted.