Defining a method inside a module in ruby (NoMethodError)
The Ruby documentation on Module answers this in its introduction text.
This form:
module Familiar def ask_age return "How old are you?" endenddefines #ask_age as an instance method on Familiar. However, you can't instantiate Modules, so you can't get to their instance methods directly; you mix them into other classes. Instance methods in modules are more or less unreachable directly.
This form, by comparison:
module Familiar def self.ask_age return "What's up?" endenddefines ::ask_age as a module function. It is directly callable, and does not appear on included classes when the module is mixed into another class.