“Linux From Scratch (LFS) is a project that provides you with step-by-step instructions for building your own custom Linux system, entirely from source code.”
If you wanna be a hardcore UNIX/Linux system administrator, and a lot of free time to learn stuff during a holiday, LFS is the way to go IMHO 😉
I need to try this someday to improve what I learned from playing with, and using, Gentoo Linux 😀