Is a door lock a finite state machine?
Yes and no. Arguably, the lock is not a finite state machine, since the key can be inserted to any degree and, if it fits, rotated through any angle. However, we can abstract away these unimportant details, yes, the system can be modeled as a finite number of states: essentially, if you code up the ridges of the key as symbols in some alphabet, the lock is a DFA that accepts exactly one word (or perhaps more, if there's a master key).