The fifty-year old son was complaining to his 75 year old father: "Dad, lately I've been having a problem with impotence. Tell me, do you have the same problem? Is it something that runs in our family?"
"Well," his father replied, "I don't know about your mother's side of the family, but I've never had reason to complain. Why, when I was a lad, I would work up a good hard-on, hang a bucket of water from it, and walk the full length of a football field. But nowadays my knees give in when I'm half way."