Whats ups & How R U's

Have you ever really thought about the questions or rather greetings we say when we see someone? What's Up and How are you are the most common. I personally always have a hard time with these questions, I mean are we all supposed to give the casual, "nothing" or does that person really care and is wondering? I never know what to say when i am asked one of those questions. I usaually just say nothing but then i think about it and if i need soemone to talk to, Now is my chance. But then again we never really know if the person is expecting a REAL answer or is expecting just the casual, "nothing." This is something I will never really know for sure, I guess it all depends on the person.