Take in a boarder
An elderly man tells the Doctor he is planning on marrying a women of 30, and would he have any suggestions. "Yes," says the Doctor, "I would advise you to take in a boarder." A year later at his 80th year check-up, the Doctor asks how everything is going. He says fine his wife is pregnant. The Doctor remarks: "so you took my advise and took in a boarder?" "Yes I did, is the reply, and she's pregnant also....."