From his post:
The NSObject class lies at the root of (almost) all classes we build and use as part of Cocoa programming. What does it actually do, though, and how does it do it? Today, I'm going to rebuild NSObject from scratch, as suggested by friend of the blog and occasional guest author Gwynne Raskind.
He goes through memory management, inheritance, performing selectors, etc. Most of the heavy lifting is done by the runtime. It's really amazing to see how simple it is to make this. I know I've thought the source code for NSObject must some crazy magic.