Imagine standing in the frozen-food aisle of your supermarket, staring at a tub of your favorite double-fudge brownie ice cream. Suddenly, a voice comes out of nowhere to tell you there's a two-for-one sale on that very treat. But only you hear the message. In fact, the guy studying the cookie-dough choices 2 feet away is listening to a different ad―which you can't hear.
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