Old man goes into a fishmongers.
He says "I'm 80years old, I've just married a 20 year old and I need some help performance-wise. I hear oysters do the trick."
"Always do the trick" replies the fishmonger.
"Right I'll have a dozen".
Next morning the old man bursts into the fishmongers, waving his stick and fuming.
"I demand my money back!" he shouts.
"Why what's the problem?" asks the fishmonger.
"Only eleven of 'em worked!"