Seems the interviewer has a set of question on his laptop and started shooting them one by one. When I explained the runtime (sounds for him, "ohh does Obj-C supports runtime too!") he got totally confused. I'm an hacker and always play the stuff with my jailbreak device. When he asked "How can we declare a method as private?" I explain that "you can't, since the dynamic nature of the Obj-C." And I explained how the "messages" passed to the objects at the run time. Finally he got confused and he is really looking for the answer "through extensions." (of course I told him at last by seeing his confusion face). Come on, from 10+ years of experienced guy you can't expect such a silly answer.