Whew!!

An old couple are taking a trip down memory lane by going back to the place where they first met. Sitting in a cafe, the little old man says, "Remember the first time I met you over 50 years ago? We left this cafe, went round the corner behind the gas works, and I gave you one from behind."
"Why, yes, I remember it well, dear," replies the little old lady with a grin.
"Well", said the little old man, "for old time's sake, let's go there again, and I'll give you one from behind."
Without them knowing, the young man sitting next to them had overheard the conversation and smiled to himself, thinking it would be quite amusing to see two old folks going at it.
He gets up and follows them. Sure enough, he sees the old couple near the gas works. The little old lady pulls off her underwear and lifts up her dress. The old man pulls down his pants and grabs the lady's hips, the little old lady reaches for the fence. What follows is 40 minutes of the most athletic sex the young man has ever seen. The little old man is banging away at the little old woman at a pace that can only be described as phenomenal. Limbs are flying everywhere, the movement is a blur, and they do not stop for a single second.
Finally, they collapse and don't move for an hour.
The young man is stunned. Never in his life has he ever seen anything that equates to this - not in the movies, not from his friends, not from his own experiences. Reflecting on what he has just seen, he says to himself, "I have to know his secret. If only I could fuck like that now, let alone in 50 years time!"
By this time the old folks have recovered and dressed themselves.
Plucking up courage, the young man approaches the couple. He says, "Sir, in all my life I have never seen anybody shag like that, particularly at your age. What's your secret? Could you do it like that 50 years ago?"
The old man replies, "Son, 50 years ago, that fucking fence wasn't electrified!"