For example, a Dog is a Mammal because dogs inherit certain characteristics from Mammals, such as mammary glands and (through Mammal's parent, Vertebrate) a backbone. On the other hand, dogs also may have one of several distinct types of behavior, and these behaviours may change over time. For example, a Dog may be a Pet, a Stray (an abandoned pet will acquire behaviours to survive not associated with a pet), or a Guide for the blind (guide dogs are trained, so they do not start life as guide dogs). However, these are sets of additional behaviors that can be added to a Dog. It is also possible to describe these behaviors in such a way that they can be usefully applied to other animals, for example, a Cat can equally be a Pet or Stray. Hence, Dog and Cat are both distinct from each other, while both remaining within the more general category Mammal. So Mammal is a Class and Dog and Cat are classes that inherit from Mammal. But the behaviours associated with Pet, Stray, and Guide are Roles that can be added to Classes, or objects instantiated from Classes.