An Ad Rule a day

Do you want to know the universal rules behind successful ads? I shall start a series called "An Ad Rule a Day" to introduce an advertising rule taken from the ad gurus in the world.  I shall post an advertising rule everyday for your reading and to provoke your thinking.  But I need all of you to response to it in order to keep me going.  So if you want to learn more, you should write something about the rule on that day.  Anything will do, my objective is to help you learn and think critically.  Watch out!