There is a story of a Persian mystic and humorist.
Once he was invited to give a talk. He came and started saying, "I'm sure you all know what I'm going to say."
The people said, "Yes we know." The mystic said, "Then what is the point in my talking? I'm going home."
The people realized their folly and invited him again.
Again the man started saying "You all know what I'm going to say", to which, this time, the people said, "No, we don't know."
"Then there is no point in my telling you anything", he said and went home.
They realized what their mistake was and after a week they went and invited him again.
As usual he started with the same thing. Half the audience said "Yes", the other half said "No".
"Those who know will tell those who don't know", said the mystic and went home.