Harry is getting along in years and finds that he is unable to perform sexually. He finally goes to his doctor, who tries a few things but nothing seems to work. So the doctor refers him to an American Indian medicine man.

The medicine man says, "I can cure this." That said, he throws a white powder in a flame, and there is a flash with billowing blue smoke. Then hesays, "This is powerful medicine. You can only use it once a year. All you have to do is say '123' and it shall rise for as long as you wish!"

Harry then asks, "What happens when it's over, and I don't want to continue?"
The medicine man replies: "All you or your partner has to say is 1234, and it will go down. 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 man had promised. Joyce, who had been facing away from him, turns over and asks, "What did you say 123 for?"

And that, my friends, is why you shouldn't end a sentence with a preposition.