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."