Harry is getting along in years and finds that he is unable to perform. He finally goes to his doctor, who tries a few things but nothing seems to work. So eventually the doctor refers him to an old Gypsy medicine woman.
The medicine woman says, "I can cure this." That said, she throws a white powder in a flame, and there is a flash with billowing blue smoke. She collects the ash, then she says, "This is powerful medicine. You can only use it once a year. All you have to do is say 123"
The guy then asks, "What happens when it's over, and I don't want to continue?". The medicine woman replies: "All you or your partner has to say is 1234. But be warned - it will not work again for another year!"
Harry rushes home, eager to try out his new powers and prowess. That night he is ready to surprise Joyce. He showers, shaves, and puts on his most exotic shaving lotion. He gets into bed, and lying next to her says, "123." He suddenly becomes more aroused than anytime in his life just as the medicine woman had promised.
Joyce, who had been facing away, turns over and asks, "What did you say 123 for?"
And that, my friends, is why you should never end a sentence with a preposition.