A method and apparatus for dynamically modifying search results “on the fly” based on the behavior of the user currently conducting a search. In one embodiment, data regarding user behavior is gathered from virtually any activity, including clicks on links, dwell times, downloads, transactions and cursor movements. Subordinate keywords are generated to reflect the intent of the user as inferred from the user's behavior. Subordinate keywords, as opposed to traditional primary keywords, are keywords that are identified as important to the search, but are not necessarily essential for a matched document. They are automatically generated by the system from a variety of places, such as documents clicked on by the user as well as documents that are skipped. The system uses the subordinate keywords to dynamically re-rank thatched documents and advertisements to best reflect the inferred intent of the user in order to continuously provide the most relevant results.