After being married a year, the couple went to the hospital for the birth of their first child.
The attending nurse came out of the delivery room to congratulate the old gentleman and said, "This is amazing.
How do you do it at your age?"
The old man grinned and said, "You got to keep the old motor running."
The following year, the couple returned to the hospital  for the birth of their second child. The same nurse was attending the delivery and again went out to congratulate the old gentleman.
She said, "Sir, you are something else. How do you manage it?"
The old man grinned and said, "You gotta keep the old motor running."
A year later, the couple returned to the hospital for the birth of their third child.
The same nurse was there for this birth also and after the delivery, she once again approached the old gentleman, smiled, and said, "Well, you surely are something else! How do you do it?"
The old man replied, "It’s like I’ve told you before, you got to keep the old motor running."
The nurse, still smiling, patted him on the back and said:
"Well, I guess it’s time to change the oil. This one’s black."