The eighty year old man had gone to see his doctor for pre-marriage tests.
"I'm marrying a twenty year old", he said.
"Why are you doing that?", asked the doctor.
"I want a son and heir. Can you give me any advice?"
"Yes, get a lodger", said the doctor, smirking. A few months later, the old fellow returned to the doctor.
"Is your wife pregnant yet?", queried the doctor.
"Yes".
"So you did take in a lodger?"
"Oh yes", replied the old man, "and she's pregnant too!"