I understand the idea that someone might feel as though they might be beating their head against solid rock when they are trying to explain something to another individual. Maybe that is one of the reasons Jesus chose 12 men to begin with. Whereas one person might not be able to explain something and be understood by using multiple words and phrases another can simply walk up and say a dozen words or so and be totally understood by a questioning individual. The giver and the receiver have to be on the same wave length it would seem, and if they are not then someone else should try the match-up in order to avoid confusion and a headache. ( we all know who the author of confusion is, correct?) Peter is called a rock because I believe he would hit you over the head with one in order to be understood and make you believe. Some people need that, while others, not so much.