An 80 year old man is on honeymoon with his 20 year old wife. Because of his loud snoring, he suggests they sleep in separate bedrooms. "And, when I want some fun," he says, with a twinkle in his eye, "I'll knock on your door." She agrees and 15 mins later, she hears a knock. He enters, they have their fun and he returns to his bedroom.
20 minutes later, he knocks again, enters her bedroom where they have their fun before he returns to his own room.
20 mins after that, he's at the door again. He enters, they have fun, this time lasting several hours. Laying there exhausted, she says to him; "Know something, babe. I've been with men a quarter of your age and none have lasted as long as you, nor have made love to me as many times in one night. What's your secret?"
To which he replies, "I'm sorry, have we met?"