He answered, " You've got to keep that old motor running." 

The following year she gave birth again. The same nurse said, "You really are amazing.How do you do it?" He again said, "You've got to keep the old motor running." 

The same thing happened the next year. The nurse said, "You must be quite a man." He responded, "You've got to keep that old motor running." 

The nurse then said, "Well, you had better change the oil. This one's black!"