Most permission tutorials start with the table. Nine bits, three triplets, r is 4, w is 2, x is 1. You read it, you nod, and a week later you are pasting chmod 777 from a forum answer like everyone else.
This one runs backwards. We break things on purpose, read the exact error each breakage produces, and work out what the system was checking. The table arrives at the end, where it stops being trivia and becomes the name for something you have already felt.
You need any Linux machine: a VM, a cheap cloud box, a Raspberry Pi, WSL on Windows. Everything happens inside one throwaway folder, and the worst outcome available to you is deleting that folder. Twenty minutes.
The three questions
Hold one model in your head the whole way. Every time you touch a file, the kernel, the core of Linux that referees every file access, asks three questions in this order:
