A retired gentleman of quite mature years befriended a younger woman in her prime, and a romance bloomed.
He proposed, she accepted, but the gent decided that a physical exam was in order before the wedding.
The doctor's verdict was "You're not the man you once were, but with clean living, you could still be here for quite a while."
"But Doctor", said his patient, "I'm about to marry a much younger woman. How can I keep her satisfied?"
"With a man of your age", said the doctor, "I'll be frank; my advice to you is to take in a young boarder".
Several months later, the two men met again on the street, and the old gent looked terrific.
"How's your new wife?" asked the Doc.
"I'm happy to say she's pregnant."
"I guess you took my advice", said the physician, "about that young boarder."
"That I did, sir, and she's pregnant too!"