don't know

Before Fifi apply for new job as an accountant in a company he was a plumber worker.On day he go for his interview.The one who interview him was his uncle.Finish the interview question,Fifi uncle tell him that he does not know two things.Fifi wondering what does his uncle mean and curiously he ask his uncle what's that so that from that day onwards he will know it.
The uncle tell him with innocent look "this you don't know,that you don't know."