A father used to say to his children when they were young: —When you all reach the age of 12 I will tell you the secret of life.
One day when the oldest turned 12, he anxiously asked his father what was the secret of life. The father replied that he was going to tell him, but that he should not reveal it to his brothers.
The secret of life is this: The cow does not give milk.
"What are you saying?" Asked the boy incredulously.