Gave a very general answer about what OO design is and gave an example i.e a class Cat can be a subclass of class Animal. Animal can have functions like walk or eat. Cats can walk and eat, but also can lick or purr. This lets Cat be organized into a bigger class of Animal, but it has it's own functions to define the behavior of the class. This makes organization and implementation easier, ie Another programmer can easily use Cat instead of Animal to describe a Cat.