Originally Posted by Larry Delor

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." With that said, he
throws a white powder in a flame, and there is a flash with billowing blue smoke.

Then he says, "This is powerful medicine but 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!" The guy 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."

That night he is ready to surprise his wife. He showers, shaves,
and puts on his most exotic shaving lotion and cologne. After he gets
into bed and is lying next to her, he says, "123", and suddenly he becomes
more aroused than anytime in his life, just as the medicine man had promised.

His wife, who had been facing away from him, turns over and asks,
"What did you say 123 for?"

And now you know why you shouldn't end a sentence with a preposition.