“A hook is a technique that lets you trap some Ruby event, such as object creation. The simplest hook technique in Ruby is to intercept calls to methods in system classes.” - Programming Ruby, Dave Thomas

Module#method_added
Module#method_removed
Module#method_undefined
Module#extend_object

Kernel.singleton_method_added
Kernel.singleton_method_undefined

Class.inherited … gets called by any subclass of this class.