A traditional rule of grammar is that one should never end a sentence with a preposition. Facetiously stated, the rule is, "A preposition is something you should never end a sentence with." Although it is generally advisable to structure sentences so that they do not end in prepositions, as this makes for more elegant writing, many dispute that ending a sentence with a preposition is incorrect English, especially when there is no convenient way to reword the sentence.
Sometimes the "correct" wording is humorously awkward, as in, "Mr. Hunter cursed his memory of the milkman, away with which his wife ran."
Winston Churchill once put a preposition at the end of a sentence and was called to task for it. As the story goes, Churchill replied, "That's the sort of pedantry up with which I will not put."
Another interesting sentence that plays with sentence-end prepositions is, "Aw, Mom, what'd you bring that book I don't like to be read to out of up for?" If the book in question was about Australia, the number of prepositions at the end can be increased from five to eight: "Aw, Mom, what'd you bring that book I don't like to be read to out of about Down Under up for?" "Down Under" is used in this sentence as a single noun rather than as two prepositions, but we needn't let a technicality like that ruin our fun.