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Our classification of objects in everyday life is naturally hierarchical. We know that all cats are mammals, and all mammals are animals. Smaller classes inherit characteristics from the larger classes to which they belong. If all mammals breathe, then all cats breathe.

We can express this concept in ruby:

ruby> class Mammal
    |   def breathe
    |     print "inhale and exhale\n"
    |   end
    | end
   nil
ruby> class Cat<Mammal
    |   def speak
    |     print "Meow\n"
    |   end
    | end
   nil

Though we didn't specify how a Cat should breathe, every cat will inherit that behavior from the Mammal class since Cat was defined as a subclass of Mammal. (In OO terminology, the smaller class is a subclass and the larger class is a superclass.) Hence from a programmer's standpoint, cats get the ability to breathe for free; after we add a speak method, our cats can both breathe and speak.

ruby> tama = Cat.new
   #<Cat:0xbd80e8>
ruby> tama.breathe
inhale and exhale
   nil
ruby> tama.speak
Meow
   nil

There will be situations where certain properties of the superclass should not be inherited by a particular subclass. Though birds generally know how to fly, penguins are a flightless subclass of birds.

ruby> class Bird
    |   def preen
    |     print "I am cleaning my feathers."
    |   end
    |   def fly
    |     print "I am flying."
    |   end
    | end
   nil
ruby> class Penguin<Bird
    |   def fly
    |     fail "Sorry. I'd rather swim."
    |   end
    | end
   nil

Rather than exhaustively define every characteristic of every new class, we need only to append or to redefine the differences between each subclass and its superclass. This use of inheritance is sometimes called differential programming. It is one of the benefits of object-oriented programming.

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