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....."