Invisible Hand Gestures

Why is it when people talk on their mobile phones (or any phones for that matter) they make wild hand gestures to stress their point, even silly faces and end up getting really rather animated! Its so much fun just watching it. There was this visitor in reception this morning who just made me giggle becuase he was getting rather excited explaining some kind of business solution to a client down the other end of the phone. His hands were all over the place, and I was transfixed as he just didn't realise at all what he was doing! It was so funny, made me giggle! I mean, I'm sure I do exactly the same thing, even when I know that no one can see me! Must look a complete sight, you know, this human need for communication, verbal and non-verbal!