"Is it in His Knee? No the Magic is in the Fence" joke

An elderly couple is enjoying an anniversary dinner together in a small tavern. The husband leans over and ask his wife .
Do you remember the first time we had sex together over fifty years ago? We went behind this tavern where you leaned against the fence and I made love to you.
"Yes", she said, "I remember it well."
"Ok", he says "How about taking a stroll around there again and do it for old time sake ."
"Ooooooh Henry, you devil, that sounds like a good idea", she answers.
There was a police officer sitting in the next booth listening to all this and having a chuckle to himself. he thinks,"I've got to see this, two old-timers having sex against a fence, I'll just keep an eye on them so there's no trouble. He follows them...
They walked haltingly along, leaning on each other for support, aided by walking sticks, Finally they get to the back of the tavern and make thier way to the fence. The old lady lifts her skirt, pulls her knickers down and the old man drops his trousers, the old man moves in, suddenly they erupt into the most furoius sex that the watching policeman has ever seen .
They are bucking and jumping like eighteen year olds. this goes on for about forty minutes! She's yelling "Ohhhh, God' he's hanging on to her hips for dear life. This is the most athletic sex imaginable. Finally they both collapse panting on the ground.
After about an hour of lying on the ground recovering, the old couple struggle to thier feet and put their clothes back on.
The police man quite amazed stood there still watching and still thinking to himself . " That was truly amazing, he was like a train. I've got to ask him what his secret is."
As the couple passed, he says to them, "That was amazing, you must have been having sex for about forty minutes. How do you manage it?
You must have had a fantastic life together, Is there some sort of secret?"
"No, there's no secret" the old man says,
"fifty years ago that darn fence wasn't electrical!"