From the great Language Log:
Most of the ambiguity contained in normal language use is passed over without any awareness on the audience’s part of the potential for double meanings. If one of the two intended meanings in an ad happens to be too faint, the ad must do something to fortify it and nudge it over the threshold of awareness. For instance, an ad that ran some time ago for British Airways used the tagline “Showers expected upon arrival.” To make it clear to the reader that showers could also refer to the bathing facilities that were available in the airline’s lounge at Heathrow Airport, the advertisers resorted to a fairly crude visual technique: a photo of a showerhead accompanied the tagline.
Much more sophisticated is the use of ambiguity in the following:
Are you up in the air about your future? Maybe that’s where you belong.
The ad was placed by the U.S. Air Force, whose logo appeared in the usual lower right hand corner of the print ad. I’m guessing that a typical reader response looks something like this: The idiomatic reading of “up in the air” is far more accessible in the first sentence than the literal one, and is perhaps the only meaning that the reader registers. The second sentence prompts a reanalysis, with the anaphoric where pointing to a definite spatial location, but it’s still not really clear how this second sentence fits in coherently with the first, on either the literal or idiomatic reading—until the reader’s eye lands on the logo, and voilà—the literal reading is further primed, and the identity of the sponsor now allows the reader to fit all the pieces together.
Such manipulation of context, to render meeker meanings more assertive, is part-and-parcel of the adept use of ambiguity. …
