ITP2 - OOP Tutorials - Method Lookup

The Speak module is placed right in between our custom classes (i.e.,
GoodDog and HumanBeing) and the Object class that comes with Ruby. In
Inheritance you'll see how the method lookup chain works when working
with both mixins and class inheritance.

This means that since the speak method is not defined in the GoodDog
class, the next place it looks is the Speak module. This continues in an
ordered, linear fashion, until the method is either found, or there are
no more places to look.