"Search engine ads work because search engines are the one type of website that people visit with the explicit goal of finding someplace else to go. Thus, if users see an ad for what they're looking for, there is a high probability that they'll click that ad. Advertisers can satisfy a user's immediate needs because they target ads based on the user's query terms. (This also explains why ads on search engine homepages don't work: it's impossible to target the ad to the user's current quest until the server knows what that quest is.)" -Jakob Nielson
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