Another object-oriented model concept is also enabled by class derivation -- polymorphism (Greek: "having multiple forms").
Consider a bird's flap method. Depending on species, the flap method can be used to
or communicate with other birds.
Similarly, class derivatives can provide alternative behaviors for a common method of the base class.
The query time example uses a variety of classes that are derivatives of an abstract emitter base class.
When the emit method is performed, the specific class derivative renders the time of day using its specific technique.