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....."