It is hard to get the requisite humour across here.

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"I want to be a writer, sir", I said with complete earnesty, wide-eyes staring expectantly at him.
He had none of it, and gruffly said "To write, you have to know people, understand them".
After a slight pause, I excitedly quipped "Ah, but I know myself!", and half-imagined I had broken his defense.
But he just smiled at me, and clinched victory with the wry words "Then perhaps you know why it will not be".