Contrary to popular opinion, no rule states that it is incorrect to end a sentence with a preposition.  A principle of style, however, declares that one should not end a sentence with a preposition when one has a graceful alternative.  As Theodore M. Bernstein says in The Careful Writer (Atheneum: 1968), "It is well to consider that a sentence ending with a preposition is sometimes clumsy, often weak."  But Bernstein adds that "a preposition can itself provide strength at the end of a sentence."  "This occurs," he says, "when the preposition carries real import and the verb has a rather low charge; in such instances heavy stress . . . falls on the preposition, and idiom declares that it appear at the end."

Bernstein's examples prove the point.  How else are we to say, "He didn't know what he was getting into," "I found this tool, but I don't know what it is used for," or "I didn't know what it was all about"?  Consider, he says, Shakespeare's "We are such stuff as dreams are made on"* and such expressions as: "That is something to guard against," "He is someone you can count on," and "You don't know what I have been through."  Bernstein wryly suggests that anyone who calls such expressions wrong will find that he or she "hasn't a leg on which to stand."